imagecopyresampled()

(PHP 4 >= 4.0.6, PHP 5, PHP 7)

重采样拷贝部分图像并调整大小

说明

imagecopyresampled(resource $dst_image,resource $src_image,int $dst_x,int $dst_y,int $src_x,int $src_y,int $dst_w,int $dst_h,int $src_w,int $src_h): bool

imagecopyresampled()将一幅图像中的一块正方形区域拷贝到另一个图像中，平滑地插入像素值，因此，尤其是，减小了图像的大小而仍然保持了极大的清晰度。

In other words,imagecopyresampled() will take a rectangular area from$src_imageof width$src_wand height$src_hat position($src_x,$src_y)and place it in a rectangular area of$dst_imageof width$dst_wand height$dst_hat position($dst_x,$dst_y).

如果源和目标的宽度和高度不同，则会进行相应的图像收缩和拉伸。坐标指的是左上角。本函数可用来在同一幅图内部拷贝（如果$dst_image和$src_image相同的话）区域，但如果区域交迭的话则结果不可预知。

参数

$dst_image

目标图象连接资源。

$src_image

源图象连接资源。

$dst_x

目标 X 坐标点。

$dst_y

目标 Y 坐标点。

$src_x

源的 X 坐标点。

$src_y

源的 Y 坐标点。

$dst_w

目标宽度。

$dst_h

目标高度。

$src_w

源图象的宽度。

$src_h

源图象的高度。

返回值

成功时返回TRUE，或者在失败时返回FALSE。

范例

简单的例子

这个例子会将图像调整为原有尺寸的一半。

<?php
// 这个文件
$filename = 'test.jpg';
$percent = 0.5;
// 内容类型
header('Content-Type: image/jpeg');
// 获取新的尺寸
list($width, $height) = getimagesize($filename);
$new_width = $width * $percent;
$new_height = $height * $percent;
// 重新取样
$image_p = imagecreatetruecolor($new_width, $new_height);
$image = imagecreatefromjpeg($filename);
imagecopyresampled($image_p, $image, 0, 0, 0, 0, $new_width, $new_height, $width, $height);
// 输出
imagejpeg($image_p, null, 100);
?>

以上例程的输出类似于：

按比例对图像重新采样

这个例子会以最大宽度高度为 200 像素显示一个图像。

<?php
// 源文件
$filename = 'test.jpg';
// 设置最大宽高
$width = 200;
$height = 200;
// Content type
header('Content-Type: image/jpeg');
// 获取新尺寸
list($width_orig, $height_orig) = getimagesize($filename);
$ratio_orig = $width_orig/$height_orig;
if ($width/$height > $ratio_orig) {
   $width = $height*$ratio_orig;
} else {
   $height = $width/$ratio_orig;
}
// 重新取样
$image_p = imagecreatetruecolor($width, $height);
$image = imagecreatefromjpeg($filename);
imagecopyresampled($image_p, $image, 0, 0, 0, 0, $width, $height, $width_orig, $height_orig);
// 输出
imagejpeg($image_p, null, 100);
?>

以上例程的输出类似于：

注释

Note:

因为调色板图像限制（255+1 种颜色）有个问题。重采样或过滤图像通常需要多于 255 种颜色，计算新的被重采样的像素及其颜色时采用了一种近似值。对调色板图像尝试分配一个新颜色时，如果失败我们选择了计算结果最接近（理论上）的颜色。这并不总是视觉上最接近的颜色。这可能会产生怪异的结果，例如空白（或者视觉上是空白）的图像。要跳过这个问题，请使用真彩色图像作为目标图像，例如用imagecreatetruecolor()创建的。

参见

imagecopyresized() 拷贝部分图像并调整大小

FOUR RECTANGLES
         $src_image                  $dst_image
+------------+---------------------------------+  +------------+--------------------+
|      |                 |  |      |          |
|      |                 |  |     $dst_y         |
|      |                 |  |      |          |
|     $src_y                |  +-- $dst_x --+----$dst_width----+ |
|      |                 |  |      |         | |
|      |                 |  |      |  Resampled   | |
|      |                 |  |      |         | |
+-- $src_x --+------ $src_width ------+    |  |    $dst_height       | |
|      |            |    |  |      |         | |
|      |            |    |  |      |         | |
|      |            |    |  |      |         | |
|      |            |    |  |      +------------------+ |
|      |    Sample     |    |  |                 |
|      |            |    |  |                 |
|      |            |    |  |                 |
|    $src_height          |    |  |                 |
|      |            |    |  +---------------------------------+
|      |            |    |
|      |            |    |
|      +------------------------+    |
|                       |
|                       |
+----------------------------------------------+

Here is my ultimate image resizer that preserves transparency for gif's and png's and has an option to crop images to fixed dimensions (preserves image proportions by default)
<?php
function image_resize($src, $dst, $width, $height, $crop=0){
 if(!list($w, $h) = getimagesize($src)) return "Unsupported picture type!";
 $type = strtolower(substr(strrchr($src,"."),1));
 if($type == 'jpeg') $type = 'jpg';
 switch($type){
  case 'bmp': $img = imagecreatefromwbmp($src); break;
  case 'gif': $img = imagecreatefromgif($src); break;
  case 'jpg': $img = imagecreatefromjpeg($src); break;
  case 'png': $img = imagecreatefrompng($src); break;
  default : return "Unsupported picture type!";
 }
 // resize
 if($crop){
  if($w < $width or $h < $height) return "Picture is too small!";
  $ratio = max($width/$w, $height/$h);
  $h = $height / $ratio;
  $x = ($w - $width / $ratio) / 2;
  $w = $width / $ratio;
 }
 else{
  if($w < $width and $h < $height) return "Picture is too small!";
  $ratio = min($width/$w, $height/$h);
  $width = $w * $ratio;
  $height = $h * $ratio;
  $x = 0;
 }
 $new = imagecreatetruecolor($width, $height);
 // preserve transparency
 if($type == "gif" or $type == "png"){
  imagecolortransparent($new, imagecolorallocatealpha($new, 0, 0, 0, 127));
  imagealphablending($new, false);
  imagesavealpha($new, true);
 }
 imagecopyresampled($new, $img, 0, 0, $x, 0, $width, $height, $w, $h);
 switch($type){
  case 'bmp': imagewbmp($new, $dst); break;
  case 'gif': imagegif($new, $dst); break;
  case 'jpg': imagejpeg($new, $dst); break;
  case 'png': imagepng($new, $dst); break;
 }
 return true;
}
?>
Example that I use when uploading new images to the server.
This saves the original picture in the form:
original.type
and creates a new thumbnail:
100x100.type
<?php
 $pic_type = strtolower(strrchr($picture['name'],"."));
 $pic_name = "original$pic_type";
 move_uploaded_file($picture['tmp_name'], $pic_name);
 if (true !== ($pic_error = @image_resize($pic_name, "100x100$pic_type", 100, 100, 1))) {
  echo $pic_error;
  unlink($pic_name);
 }
 else echo "OK!";
?>
Cheers!

I've created a PHP5 image resize class, using ImageCopyResampled, that someone might find useful, with support for JPEG, PNG, and GIF formats. It retains the original image's aspect ratio when resizing, and doesn't resize or resample if the original width and height is smaller then the desired resize.
<?php
// Imaging
class imaging
{
  // Variables
  private $img_input;
  private $img_output;
  private $img_src;
  private $format;
  private $quality = 80;
  private $x_input;
  private $y_input;
  private $x_output;
  private $y_output;
  private $resize;
  // Set image
  public function set_img($img)
  {
    // Find format
    $ext = strtoupper(pathinfo($img, PATHINFO_EXTENSION));
    // JPEG image
    if(is_file($img) && ($ext == "JPG" OR $ext == "JPEG"))
    {
      $this->format = $ext;
      $this->img_input = ImageCreateFromJPEG($img);
      $this->img_src = $img;
      
    }
    // PNG image
    elseif(is_file($img) && $ext == "PNG")
    {
      $this->format = $ext;
      $this->img_input = ImageCreateFromPNG($img);
      $this->img_src = $img;
    }
    // GIF image
    elseif(is_file($img) && $ext == "GIF")
    {
      $this->format = $ext;
      $this->img_input = ImageCreateFromGIF($img);
      $this->img_src = $img;
    }
    // Get dimensions
    $this->x_input = imagesx($this->img_input);
    $this->y_input = imagesy($this->img_input);
  }
  // Set maximum image size (pixels)
  public function set_size($size = 100)
  {
    // Resize
    if($this->x_input > $size && $this->y_input > $size)
    {
      // Wide
      if($this->x_input >= $this->y_input)
      {
        $this->x_output = $size;
        $this->y_output = ($this->x_output / $this->x_input) * $this->y_input;
      }
      // Tall
      else
      {
        $this->y_output = $size;
        $this->x_output = ($this->y_output / $this->y_input) * $this->x_input;
      }
      // Ready
      $this->resize = TRUE;
    }
    // Don't resize
    else { $this->resize = FALSE; }
  }
  // Set image quality (JPEG only)
  public function set_quality($quality)
  {
    if(is_int($quality))
    {
      $this->quality = $quality;
    }
  }
  // Save image
  public function save_img($path)
  {
    // Resize
    if($this->resize)
    {
      $this->img_output = ImageCreateTrueColor($this->x_output, $this->y_output);
      ImageCopyResampled($this->img_output, $this->img_input, 0, 0, 0, 0, $this->x_output, $this->y_output, $this->x_input, $this->y_input);
    }
    // Save JPEG
    if($this->format == "JPG" OR $this->format == "JPEG")
    {
      if($this->resize) { imageJPEG($this->img_output, $path, $this->quality); }
      else { copy($this->img_src, $path); }
    }
    // Save PNG
    elseif($this->format == "PNG")
    {
      if($this->resize) { imagePNG($this->img_output, $path); }
      else { copy($this->img_src, $path); }
    }
    // Save GIF
    elseif($this->format == "GIF")
    {
      if($this->resize) { imageGIF($this->img_output, $path); }
      else { copy($this->img_src, $path); }
    }
  }
  // Get width
  public function get_width()
  {
    return $this->x_input;
  }
  // Get height
  public function get_height()
  {
    return $this->y_input;
  }
  // Clear image cache
  public function clear_cache()
  {
    @ImageDestroy($this->img_input);
    @ImageDestroy($this->img_output);
  }
}
##### DEMO #####
// Image
$src = "myimage.jpg";
// Begin
$img = new imaging;
$img->set_img($src);
$img->set_quality(80);
// Small thumbnail
$img->set_size(200);
$img->save_img("small_" . $src);
// Baby thumbnail
$img->set_size(50);
$img->save_img("baby_" . $src);
// Finalize
$img->clear_cache();
?>

<?php
$new_file = img_resize("./img/", "test.jpg","copy_test.jpg",300);
echo "<IMG src = '$new_file'>";
function img_resize($path,$tmp_name,$new_name,$new_width){
  if (!file_exists($path.$filename)){
    echo "file not found!";
    exit;
  }
  if (!is_writable($path)){
    echo "error:permission denied!";
    exit;
  }
  list($width, $height) = getimagesize($path . $tmp_name);
  $new_height = abs($new_width * $height / $width); 
  $image_p = imagecreatetruecolor($new_width, $new_height);
  $image = imagecreatefromjpeg($path . $tmp_name); 
  imagecopyresampled($image_p, $image, 0, 0, 0, 0,
            $new_width, $new_height, $width, $height); 
  imagejpeg($image_p, $path . $new_name); 
  return $path.$new_name;
}
?>

Another add-on to previous php5 class for thumbnail (with a merge of Matt and Zorro's proposals).
This is dedicated to generate thumbnail on the fly for a webpage using the subclass thumbnail.
It saves the generated thumb as myimage_tn, in the same directory.
I'm quite new with php5, so I think this could be optimized, but it seems to work fine.
<?php
// Imaging
class imaging
{
  // Variables
  private $img_input;
  private $img_output;
  private $img_src;
  private $format;
  private $quality = 80;
  private $x_input;
  private $y_input;
  private $x_output;
  private $y_output;
  private $resize;
  // Set image
  public function set_img($img)
  {
    // Find format
    $ext = strtoupper(pathinfo($img, PATHINFO_EXTENSION));
    // JPEG image
    if(is_file($img) && ($ext == "JPG" OR $ext == "JPEG"))
    {
      $this->format = $ext;
      $this->img_input = ImageCreateFromJPEG($img);
      $this->img_src = $img;
    }
    // PNG image
    elseif(is_file($img) && $ext == "PNG")
    {
      $this->format = $ext;
      $this->img_input = ImageCreateFromPNG($img);
      $this->img_src = $img;
    }
    // GIF image
    elseif(is_file($img) && $ext == "GIF")
    {
      $this->format = $ext;
      $this->img_input = ImageCreateFromGIF($img);
      $this->img_src = $img;
    }
    // Get dimensions
    $this->x_input = imagesx($this->img_input);
    $this->y_input = imagesy($this->img_input);
  }
  // Set maximum image size (pixels)
  public function set_size($max_x = 100,$max_y = 100)
  {
    // Resize
    if($this->x_input > $max_x || $this->y_input > $max_y)
    {
      $a= $max_x / $max_y;
      $b= $this->x_input / $this->y_input;
      if ($a<$b)
      {
        $this->x_output = $max_x;
        $this->y_output = ($max_x / $this->x_input) * $this->y_input;
      }
      else
      {
        $this->y_output = $max_y;
        $this->x_output = ($max_y / $this->y_input) * $this->x_input;
      }
      // Ready
      $this->resize = TRUE;
    }
    // Don't resize    
    else { $this->resize = FALSE; }
  }
  // Set image quality (JPEG only)
  public function set_quality($quality)
  {
    if(is_int($quality))
    {
      $this->quality = $quality;
    }
  }
  // Save image
  public function save_img($path)
  {
    // Resize
    if($this->resize)
    {
      $this->img_output = ImageCreateTrueColor($this->x_output, $this->y_output);
      ImageCopyResampled($this->img_output, $this->img_input, 0, 0, 0, 0, $this->x_output, $this->y_output, $this->x_input, $this->y_input);
    }
    // Save JPEG
    if($this->format == "JPG" OR $this->format == "JPEG")
    {
      if($this->resize) { imageJPEG($this->img_output, $path, $this->quality); }
      else { copy($this->img_src, $path); }
    }
    // Save PNG
    elseif($this->format == "PNG")
    {
      if($this->resize) { imagePNG($this->img_output, $path); }
      else { copy($this->img_src, $path); }
    }
    // Save GIF
    elseif($this->format == "GIF")
    {
      if($this->resize) { imageGIF($this->img_output, $path); }
      else { copy($this->img_src, $path); }
    }
  }
  // Get width
  public function get_width()
  {
    return $this->x_input;
  }
  // Get height
  public function get_height()
  {
    return $this->y_input;
  }
  // Clear image cache
  public function clear_cache()
  {
    @ImageDestroy($this->img_input);
    @ImageDestroy($this->img_output);
  }
}
class thumbnail extends imaging {
  private $image;
  private $width;
  private $height;
  
  function __construct($image,$width,$height) {
parent::set_img($image);
parent::set_quality(80);
parent::set_size($width,$height);
      $this->thumbnail= pathinfo($image, PATHINFO_DIRNAME).pathinfo($image, PATHINFO_FILENAME).'_tn.'.pathinfo($image, PATHINFO_EXTENSION);
parent::save_img($this->thumbnail);
parent::clear_cache();
    }
  function __toString() {
      return $this->thumbnail;
  }
}
********
* DEMO *
********
$thumb = new thumbnail('./image_dir/sub_dir/myimage.jpg',100,100);
echo '<img src=\''.$thumb.'\' alt=\'myimage\' title=\'myimage\'/>';
  ?>

Okay so I saw that others have posted this already but I just messed around and made it so figured I would share it.
Mine works, not to say others don't, I haven't tested them.
This function creates a thumbnail of the specfied size.
If the thumbnail is a different ration it will automatically crop the center of the source image.
It works if the source image is bigger or smaller than the desired "cropped thumbnail."
Here is the code...
<?php
function CroppedThumbnail($imgSrc,$thumbnail_width,$thumbnail_height) { //$imgSrc is a FILE - Returns an image resource.
  //getting the image dimensions 
  list($width_orig, $height_orig) = getimagesize($imgSrc);  
  $myImage = imagecreatefromjpeg($imgSrc);
  $ratio_orig = $width_orig/$height_orig;
  
  if ($thumbnail_width/$thumbnail_height > $ratio_orig) {
    $new_height = $thumbnail_width/$ratio_orig;
    $new_width = $thumbnail_width;
  } else {
    $new_width = $thumbnail_height*$ratio_orig;
    $new_height = $thumbnail_height;
  }
  
  $x_mid = $new_width/2; //horizontal middle
  $y_mid = $new_height/2; //vertical middle
  
  $process = imagecreatetruecolor(round($new_width), round($new_height)); 
  
  imagecopyresampled($process, $myImage, 0, 0, 0, 0, $new_width, $new_height, $width_orig, $height_orig);
  $thumb = imagecreatetruecolor($thumbnail_width, $thumbnail_height); 
  imagecopyresampled($thumb, $process, 0, 0, ($x_mid-($thumbnail_width/2)), ($y_mid-($thumbnail_height/2)), $thumbnail_width, $thumbnail_height, $thumbnail_width, $thumbnail_height);
  imagedestroy($process);
  imagedestroy($myImage);
  return $thumb;
}
//Create the thumbnail
$newThumb = CroppedThumbnail("MyImageName.jpg",75,100);
// And display the image...
header('Content-type: image/jpeg');
imagejpeg($newThumb);
?>

I hated that enlarging an image resulted in giant pixels rather than a smoother look, so I wrote this function. It takes longer, but gives a much nicer look.
<?php
function imagecopyresampledSMOOTH(&$dst_img, &$src_img, $dst_x, $dst_y, $src_x, $src_y, $dst_w, $dst_h, $src_w, $src_h, $mult=1.25){
  // don't use a $mult that's too close to an int or this function won't make much of a difference
  $tgt_w = round($src_w * $mult);
  $tgt_h = round($src_h * $mult);
  
  // using $mult <= 1 will make the current step w/h smaller (or the same), don't allow this, always resize by at least 1 pixel larger
  if($tgt_w <= $src_w){ $tgt_w += 1; }
  if($tgt_h <= $src_h){ $tgt_h += 1; }
  
  // if the current step w/h is larger than the final height, adjust it back to the final size
  // this check also makes it so that if we are doing a resize to smaller image, it happens in one step (since that's already smooth)
  if($tgt_w > $dst_w){ $tgt_w = $dst_w; }
  if($tgt_h > $dst_h){ $tgt_h = $dst_h; }
  $tmpImg = imagecreatetruecolor($tgt_w, $tgt_h);
  imagecopyresampled($tmpImg, $src_img, 0, 0, $src_x, $src_y, $tgt_w, $tgt_h, $src_w, $src_h);
  imagecopy($dst_img, $tmpImg, $dst_x, $dst_y, 0, 0, $tgt_w, $tgt_h);
  imagedestroy($tmpImg);
  // as long as the final w/h has not been reached, reep on resizing
  if($tgt_w < $dst_w OR $tgt_h < $dst_h){
    imagecopyresampledSMOOTH($dst_img, $dst_img, $dst_x, $dst_y, $dst_x, $dst_y, $dst_w, $dst_h, $tgt_w, $tgt_h, $mult);
  }
}
?>

I was looking around and couldn't find a function that resizes images to any ratio without leaving a blank area, so i wrote this one. It's able to resize images to any size ratio, when the ratio is no match with the original it will crop proportional area on the original and resize it.
<?php
function _ckdir($fn) {
  if (strpos($fn,"/") !== false) {
    $p=substr($fn,0,strrpos($fn,"/"));
    if (!is_dir($p)) {
      _o("Mkdir: ".$p);
      mkdir($p,777,true);
    }
  }
}
function img_resizer($src,$quality,$w,$h,$saveas) {
  /* v2.5 with auto crop */
  $r=1;
  $e=strtolower(substr($src,strrpos($src,".")+1,3));
  if (($e == "jpg") || ($e == "peg")) {
    $OldImage=ImageCreateFromJpeg($src) or $r=0;
  } elseif ($e == "gif") {
    $OldImage=ImageCreateFromGif($src) or $r=0;
  } elseif ($e == "bmp") {
    $OldImage=ImageCreateFromwbmp($src) or $r=0;
  } elseif ($e == "png") {
    $OldImage=ImageCreateFromPng($src) or $r=0;
  } else {
    _o("Not a Valid Image! (".$e.") -- ".$src);$r=0;
  }
  if ($r) {
    list($width,$height)=getimagesize($src);
    // check if ratios match
    $_ratio=array($width/$height,$w/$h);
    if ($_ratio[0] != $_ratio[1]) { // crop image
      // find the right scale to use
      $_scale=min((float)($width/$w),(float)($height/$h));
      // coords to crop
      $cropX=(float)($width-($_scale*$w));
      $cropY=(float)($height-($_scale*$h));  
      
      // cropped image size
      $cropW=(float)($width-$cropX);
      $cropH=(float)($height-$cropY);
      
      $crop=ImageCreateTrueColor($cropW,$cropH);
      // crop the middle part of the image to fit proportions
      ImageCopy(
        $crop,
        $OldImage,
        0,
        0,
        (int)($cropX/2),
        (int)($cropY/2),
        $cropW,
        $cropH
      );
    }
    
    // do the thumbnail
    $NewThumb=ImageCreateTrueColor($w,$h);
    if (isset($crop)) { // been cropped
      ImageCopyResampled(
        $NewThumb,
        $crop,
        0,
        0,
        0,
        0,
        $w,
        $h,
        $cropW,
        $cropH
      );
      ImageDestroy($crop);
    } else { // ratio match, regular resize
      ImageCopyResampled(
        $NewThumb,
        $OldImage,
        0,
        0,
        0,
        0,
        $w,
        $h,
        $width,
        $height
      );
    }
    _ckdir($saveas);
    ImageJpeg($NewThumb,$saveas,$quality);
    ImageDestroy($NewThumb);
    ImageDestroy($OldImage);
  }
  return $r;
}
?>

The ultimate smart image re-sizing routine. You provide the original image, and a desired width and/or height. The function will intelligently re-size the original image to fit, centered, within the specified dimensions. Either width or height can be omitted to have the re-size "lock" only on width or height. Fantastic for thumbnail generation. Designed to work with PNG's.
function resize_image_smart($orig, $dest_width=null, $dest_height=null)
{
 $orig_width = imagesx($orig);
 $orig_height = imagesy($orig);
 $vertical_offset = 0;
 $horizontal_offset = 0;
 if($dest_width == null)
 {
  if($dest_height == null)
  {
   die('$dest_width and $dest_height cant both be null!');
  }
  // height is locked
  $dest_width = $dest_height * $orig_width / $orig_height;
 } else {
  if($dest_height == null)
  {
   // width is locked
   $dest_height = $dest_width * $orig_height / $orig_width;
  } else {
   // both dimensions are locked
   $vertical_offset = $dest_height - ($orig_height * $dest_width) / $orig_width;
   $horizontal_offset = $dest_width - ($dest_height * $orig_width) / $orig_height;
   if($vertical_offset < 0) $vertical_offset = 0;
   if($horizontal_offset < 0) $horizontal_offset = 0;
  }
 }
 $img = imagecreatetruecolor($dest_width, $dest_height);
 imagesavealpha($img, true);
 imagealphablending($img, false);
 $transparent = imagecolorallocatealpha($img, 0, 0, 0, 127);
 imagefill($img, 0, 0, $transparent);
 imagecopyresampled($img, $orig, round($horizontal_offset / 2),
                 round($vertical_offset / 2),
                 0,
                 0,
                 round($dest_width - $horizontal_offset),
                 round($dest_height - $vertical_offset),
                 $orig_width,
                 $orig_height);
 return $img;
}

This is a fixed version of ImageCopyResampledBicubic posted by liviu.malaescu
The original version wasn't respecting src_x & src_y args
<?php
  function ImageCopyResampledBicubic(&$dst_image, &$src_image, $dst_x, $dst_y, $src_x, $src_y, $dst_w, $dst_h, $src_w, $src_h) {
    // we should first cut the piece we are interested in from the source
    $src_img = ImageCreateTrueColor($src_w, $src_h);
    imagecopy($src_img, $src_image, 0, 0, $src_x, $src_y, $src_w, $src_h);
    // this one is used as temporary image
    $dst_img = ImageCreateTrueColor($dst_w, $dst_h);
    ImagePaletteCopy($dst_img, $src_img);
    $rX = $src_w / $dst_w;
    $rY = $src_h / $dst_h;
    $w = 0;
    for ($y = 0; $y < $dst_h; $y++) {
      $ow = $w; $w = round(($y + 1) * $rY);
      $t = 0;
      for ($x = 0; $x < $dst_w; $x++) {
        $r = $g = $b = 0; $a = 0;
        $ot = $t; $t = round(($x + 1) * $rX);
        for ($u = 0; $u < ($w - $ow); $u++) {
          for ($p = 0; $p < ($t - $ot); $p++) {
            $c = ImageColorsForIndex($src_img, ImageColorAt($src_img, $ot + $p, $ow + $u));
            $r += $c['red'];
            $g += $c['green'];
            $b += $c['blue'];
            $a++;
          }
        }
        ImageSetPixel($dst_img, $x, $y, ImageColorClosest($dst_img, $r / $a, $g / $a, $b / $a));
      }
    }
    // apply the temp image over the returned image and use the destination x,y coordinates
    imagecopy($dst_image, $dst_img, $dst_x, $dst_y, 0, 0, $dst_w, $dst_h);
    // we should return true since ImageCopyResampled/ImageCopyResized do it
    return true;
  }
?>

Having updated GD-Library from 2.0.35-r1 to 2.0.35-r3 using negative numbers for $dstX or $dstY started to produce unpredictable results, such as inserting shrinked images and enlarging $dstH or $dstW all by itself. While it did work fine previously, one should avoid using or calculating negative numbers for imagecopyresampled from the beginning.

<?php
// The file
$filein = ''; // File in
$fileout = 'ttt.gif'; // Fileout - optional 
$imagethumbsize_w = 100; // thumbnail size (area cropped in middle of image)
$imagethumbsize_h = 75; // thumbnail size (area cropped in middle of image)
resize_then_crop( $filein,$fileout,$imagethumbsize_w,
$imagethumbsize_h,/*rgb*/"255","255","255");
//Author Alan Reddan Silverarm Solutions
//Date 27/01/2005
//Function that works well with images.
//It takes the image and reduces its size to best fit. i.e If you have an image
//that is 200 X 100 and you want a thumbnail of 75 X 50,
//it first resizes the image to 100 X 50
//and then takes out a portion 75 X 50 from then center of the input image.
//So loads of image information is retained.
//The corollary also holds if your input image is 100 X 200
//it first resizes image to 75 X 150 and then takes out a
//portion 75 X 75 from the centre
// The advantage here is that function decides on whether
//resize is by width or height itself.
//it also decides whether to use the height or the width as the base start point
//in the case that athumbnail is rectangular
function resize_then_crop( $filein,$fileout,
$imagethumbsize_w,$imagethumbsize_h,$red,$green,$blue)
{
// Get new dimensions
list($width, $height) = getimagesize($filein);
$new_width = $width * $percent;
$new_height = $height * $percent;
  if(preg_match("/.jpg/i", "$filein"))
  {
    $format = 'image/jpeg';
  }
  if (preg_match("/.gif/i", "$filein"))
  {
    $format = 'image/gif';
  }
  if(preg_match("/.png/i", "$filein"))
  {
    $format = 'image/png';
  }
  
    switch($format)
    {
      case 'image/jpeg':
      $image = imagecreatefromjpeg($filein);
      break;
      case 'image/gif';
      $image = imagecreatefromgif($filein);
      break;
      case 'image/png':
      $image = imagecreatefrompng($filein);
      break;
    }
$width = $imagethumbsize_w ;
$height = $imagethumbsize_h ;
list($width_orig, $height_orig) = getimagesize($filein);
if ($width_orig < $height_orig) {
 $height = ($imagethumbsize_w / $width_orig) * $height_orig;
} else {
  $width = ($imagethumbsize_h / $height_orig) * $width_orig;
}
if ($width < $imagethumbsize_w)
//if the width is smaller than supplied thumbnail size 
{
$width = $imagethumbsize_w;
$height = ($imagethumbsize_w/ $width_orig) * $height_orig;;
}
if ($height < $imagethumbsize_h)
//if the height is smaller than supplied thumbnail size 
{
$height = $imagethumbsize_h;
$width = ($imagethumbsize_h / $height_orig) * $width_orig;
}
$thumb = imagecreatetruecolor($width , $height); 
$bgcolor = imagecolorallocate($thumb, $red, $green, $blue);  
ImageFilledRectangle($thumb, 0, 0, $width, $height, $bgcolor);
imagealphablending($thumb, true);
imagecopyresampled($thumb, $image, 0, 0, 0, 0,
$width, $height, $width_orig, $height_orig);
$thumb2 = imagecreatetruecolor($imagethumbsize_w , $imagethumbsize_h);
// true color for best quality
$bgcolor = imagecolorallocate($thumb2, $red, $green, $blue);  
ImageFilledRectangle($thumb2, 0, 0,
$imagethumbsize_w , $imagethumbsize_h , $white);
imagealphablending($thumb2, true);
$w1 =($width/2) - ($imagethumbsize_w/2);
$h1 = ($height/2) - ($imagethumbsize_h/2);
imagecopyresampled($thumb2, $thumb, 0,0, $w1, $h1,
$imagethumbsize_w , $imagethumbsize_h ,$imagethumbsize_w, $imagethumbsize_h);
// Output
//header('Content-type: image/gif');
//imagegif($thumb); //output to browser first image when testing
if ($fileout !="")imagegif($thumb2, $fileout); //write to file
header('Content-type: image/gif');
imagegif($thumb2); //output to browser
}
?>

It should be noted that the imagecopyresampled() function is much more blurry than Photoshop CS's default bicubic funtion. And looks similar to a blury version of Photoshop's bilinear function. The documentation fails to note which algorithm is used in resampling.

scale image with ratio and then completes to the canvas size by centering 
<?php
/* 
verilen maksimum değerlere göre resmi boyutlandırır ve beyaz renk ile cerceveye ortalar. trendyol entegrasyonu için yazmıştım.
Aykut YILDIZGÖRÜR 
aykuty add gmail dot com
*/
function set_image2canvas($file, $new_file, $canvas_width=1200, $canvas_height =1800, $quality=100) {
        
  if (!file_exists( $new_file )) {
    
    list($width, $height, $type, $attr) = getimagesize( $file );
    
    if($width> $height) {
        $width_tt=$canvas_width;
        $height_tt=round($height/$width*$canvas_width);
        $off_y = ceil(( $canvas_height - $height_tt)/2);
        $off_x=0;
        
    } else {
        $height_tt=$canvas_height;
        $width_tt=round($width/$height*$canvas_height);
        $off_x=ceil(($canvas_width - $width_tt)/2);
        $off_y=0;
    }
    $thumb=imagecreatefromjpeg( $file );
    $thumb_p = imagecreatetruecolor($canvas_width, $canvas_height);
    
    $bg = imagecolorallocate ( $thumb_p, 255, 255, 255 );
    imagefill ( $thumb_p, 0, 0, $bg );
    imagecopyresampled($thumb_p, $thumb, $off_x, $off_y, 0, 0, $width_tt, $height_tt, $width, $height);
    
    imagejpeg($thumb_p,$new_file,$quality);
    
  }else{ //hedef resim zaten var
      return -1;
  }
}
?>

Windows does not accept floating width and height...
So, in "Resampling an image proportionally", prefer :
<?php
if ($width/$height > $ratio_orig) {
  $width = round( $height*$ratio_orig );
} else {
  $height = round( $width/$ratio_orig );
}
?>
(used "round")
otherwise your image wont be resized.

My complete function to resize an image with exif data
<?php
function CreateThumbnail($pic,$thumb,$thumbwidth, $quality = 100)
{
     
    $im1=ImageCreateFromJPEG($pic);
    if(function_exists("exif_read_data")){
        $exif = exif_read_data($pic);
        if(!empty($exif['Orientation'])) {
        switch($exif['Orientation']) {
        case 8:
          $im1 = imagerotate($im1,90,0);
          break;
        case 3:
          $im1 = imagerotate($im1,180,0);
          break;
        case 6:
          $im1 = imagerotate($im1,-90,0);
          break;
        } 
        }
    }
    $info = @getimagesize($pic);
    
    $width = $info[0];
     
    $w2=ImageSx($im1);
    $h2=ImageSy($im1);
    $w1 = ($thumbwidth <= $info[0]) ? $thumbwidth : $info[0] ;
     
    $h1=floor($h2*($w1/$w2));
    $im2=imagecreatetruecolor($w1,$h1);
     
    imagecopyresampled ($im2,$im1,0,0,0,0,$w1,$h1,$w2,$h2); 
    $path=addslashes($thumb);
    ImageJPEG($im2,$path,$quality);
    ImageDestroy($im1);
    ImageDestroy($im2);
     
}
?>

Function to resize an image.
<?php
function resizeImage($originalImage,$toWidth,$toHeight)
{
  list($width, $height) = getimagesize($originalImage);
  $xscale=$width/$toWidth;
  $yscale=$height/$toHeight;
  if ($yscale>$xscale){
    $new_width = round($width * (1/$yscale));
    $new_height = round($height * (1/$yscale));
  }
  else {
    $new_width = round($width * (1/$xscale));
    $new_height = round($height * (1/$xscale));
  }
  
  
  $imageResized = imagecreatetruecolor($new_width, $new_height);
  $imageTmp   = imagecreatefromjpeg ($originalImage);
  imagecopyresampled($imageResized, $imageTmp, 0, 0, 0, 0, $new_width, $new_height, $width, $height);
  return $imageResized;
  
}
?>

About preserving the transparency when resizing PNGs this was the only thing that worked for me:
<?php imagealphablending($new_watermark, false);
    $color = imagecolortransparent($new_watermark, imagecolorallocatealpha($new_watermark, 0, 0, 0, 127));
    imagefill($new_watermark, 0, 0, $color);
    imagesavealpha($new_watermark, true);
    imagecopyresampled($new_watermark, $watermark, 0, 0, 0, 0, $new_watermark_width, $new_watermark_height, imagesx($watermark),imagesy($watermark)); 
?>
Tried without success:
<?php imagealphablending($new_watermark, false);
    $color = imagecolortransparent($new_watermark, imagecolorallocate($new_watermark, 0, 0, 0));
    imagefill($new_watermark, 0, 0, $color);
    imagesavealpha($new_watermark, true);
    imagecopyresampled($new_watermark, $watermark, 0, 0, 0, 0, $new_watermark_width, $new_watermark_height, imagesx($watermark),imagesy($watermark));
 ?>
hope it helps someone.

Resampling GIFs and retain transparency dosen't work always, as dependent on the resample factor the resulting resampled image has some pattern-noise that prevents "imagegif" to find all the transparent-pixels.
One way to fix this, is to reset all pixels with high alpha to full-transparent:
<?php
// load/create images
$img_src=imagecreatefromgif($g_srcfile);
$img_dst=imagecreatetruecolor($g_iw,$g_ih);
imagealphablending($img_dst, false);
// get and reallocate transparency-color
$transindex = imagecolortransparent($img_src);
if($transindex >= 0) {
 $transcol = imagecolorsforindex($img_src, $transindex);
 $transindex = imagecolorallocatealpha($img_dst, $transcol['red'], $transcol['green'], $transcol['blue'], 127);
 imagefill($img_dst, 0, 0, $transindex);
}
// resample
imagecopyresampled($img_dst, $img_src, 0, 0, 0, 0, $g_iw, $g_ih, $g_is[0], $g_is[1]);
// restore transparency
if($transindex >= 0) {
 imagecolortransparent($img_dst, $transindex);
 for($y=0; $y<$g_ih; ++$y)
  for($x=0; $x<$g_iw; ++$x)
   if(((imagecolorat($img_dst, $x, $y)>>24) & 0x7F) >= 100) imagesetpixel($img_dst, $x, $y, $transindex);
// save GIF
imagetruecolortopalette($img_dst, true, 255);
imagesavealpha($img_dst, false);
imagegif($img_dst, $g_dstfile); 
imagedestroy($img_dst); 
?>

This is a function I wrote for making thumbnails - it will accept a source *image resource* and destination *path*, plus the max dimension and whether the thumbnail should be square.
I chose to accept a resource as the source to make it a bit more efficient to create multiple sizes. For example:
<?php
$src_im=@imagecreatefromjpeg($pathtofile);
$large = resize($src_im,$destination_large,1024);
@imagedestroy($src_im);
$medium = resize($large,$destination_medium,500);
@imagedestroy($large);
$small = resize($medium,$destination_small,125);
@imagedestroy($medium);
$square = resize($small,$destination_square,75,TRUE);
@imagedestroy($small);
@imagedestroy($square);
function resize($src_im, $dpath, $maxd, $square=false) {
  $src_width = imagesx($src_im);
  $src_height = imagesy($src_im);
  $src_w=$src_width;
  $src_h=$src_height;
  $src_x=0;
  $src_y=0;
  if($square){
    $dst_w = $maxd;
    $dst_h = $maxd;
    if($src_width>$src_height){
      $src_x = ceil(($src_width-$src_height)/2);
      $src_w=$src_height;
      $src_h=$src_height;
    }else{
      $src_y = ceil(($src_height-$src_width)/2);
      $src_w=$src_width;
      $src_h=$src_width;
    }
  }else{
    if($src_width>$src_height){
      $dst_w=$maxd;
      $dst_h=floor($src_height*($dst_w/$src_width));
    }else{
      $dst_h=$maxd;
      $dst_w=floor($src_width*($dst_h/$src_height));
    }
  }
  $dst_im=@imagecreatetruecolor($dst_w,$dst_h);
  @imagecopyresampled($dst_im, $src_im, 0, 0, $src_x, $src_y, $dst_w, $dst_h, $src_w, $src_h);
  @imagejpeg($dst_im,$dpath);
  return $dst_im;
}
?>

A tiny ammendment to zorroswordsman at gmail dot com's resize class. It was only resizing if both the width AND height where greater than the desired size. This should fix it:
<?php
// Set maximum image size (pixels)
function set_size($size = 100)
{
  // Resize
  if($this->x_input > $size || $this->y_input > $size)
  {
    // Wide
    if($this->x_input >= $this->y_input)
    {
      $this->x_output = $size;
      $this->y_output = ($this->x_output / $this->x_input) * $this->y_input;
    }
    // Tall
    else
    {
      $this->y_output = $size;
      $this->x_output = ($this->y_output / $this->y_input) * $this->x_input;
    }
    // Ready
    $this->resize = TRUE;
  }
  // Don't resize
  else { $this->resize = FALSE; }
}
?>

In case you are having trouble ( like I did ) and getting some kind of noise in the resampled image.
then changing imagecopyresampled() with imagecopyresized() removes the noise, found that here: http://bugs.php.net/bug.php?id=45030
Do not ask me why. but it works.

Wrote a function for sanitising user uploaded images. It saves the native image in size roughly 800x600 so it still fits on most screens when opened, makes a desired size thumbnail and turns all images into high quality jpegs for smaller bandwidth use.
Thumbnails are made in a similar way a designer would make them in photoshop, first resize the most troublesome dimension to desired size, then crop the rest out so the image retains proportions and most of it ends up in the thumbnail.
<?php 
// $image is $_FILES[ <image name> ]
// $imageId is the id used in a database or wherever for this image
// $thumbWidth and $thumbHeight are desired dimensions for the thumbnail
function processImage( $image, $imageId, $thumbWidth, $thumbHeight )
{
  $type = $image[ 'type' ];
  $galleryPath = 'images/collection/';
  
  if ( strpos( $type, 'image/' ) === FALSE )
  { // not an image
    return FALSE;
  }
  $type = str_replace( 'image/', '', $type );
  $createFunc = 'imagecreatefrom' . $type;
  
  $im = $createFunc( $image[ 'tmp_name' ] );
  
  $size = getimagesize( $image[ 'tmp_name' ] );
  
  $w = $size[ 0 ];
  $h = $size[ 1 ];
  if ( $w > 800 || $h > 600 )
  { // we make sure the image isn't too huge
    if ( $w > 800 )
    {
      $nw = 800;
      $nh = ceil( $nw*($h/$w) );
    }elseif( $h > 600 )
    {
      $nh = 600;
      $nw = ceil( $nh*($w/$h) );
    }
    
    $im2 = imagecreatetruecolor( $nw, $nh );
    imagecopyresampled( $im2, $im, 0, 0, 0, 0, $nw, $nh, $w, $h );
    imagedestroy( $im );
    
    $im = $im2;
    $w = $nw;
    $h = $nh;
  }
  
  // create thumbnail
  $tw = $thumbWidth;
  $th = $thumbHeight;
  $imT = imagecreatetruecolor( $tw, $th );
  
  if ( $tw/$th > $th/$tw )
  { // wider
    $tmph = $h*($tw/$w);
    $temp = imagecreatetruecolor( $tw, $tmph );
    imagecopyresampled( $temp, $im, 0, 0, 0, 0, $tw, $tmph, $w, $h ); // resize to width
    imagecopyresampled( $imT, $temp, 0, 0, 0, $tmph/2-$th/2, $tw, $th, $tw, $th ); // crop
    imagedestroy( $temp );
  }else
  { // taller
    $tmpw = $w*($th/$h );
    $imT = imagecreatetruecolor( $tmpw, $th );
    imagecopyresampled( $imT, $im, 0, 0, 0, 0, $tmpw, $h, $w, $h ); // resize to height
    imagecopyresampled( $imT, $temp, 0, 0, $tmpw/2-$tw/2, 0, $tw, $th, $tw, $th ); // crop
    imagedestroy( $temp );
  }
  
  // save the image
  imagejpeg( $im, $galleryPath . $imgid . '.jpg', 100 );
  imagejpeg( $imT, $galleryPath . $imgid . '_thumb.jpg', 100 );
}
?>

A small thumb script. Lets you specify max height and width. The thumb will always be of a rectangular shape while the image itself retains it's proportions. Very clean.
<?php
// The file
$filename = 'a.jpg';
// Set a maximum height and width
$width = 80;
$height = 80;
$thumbsize = 80;
// Content type
header('Content-type: image/jpeg');
// Get new dimensions
list($width_orig, $height_orig) = getimagesize($filename);
$ratio_orig = $width_orig/$height_orig;
if ($width/$height > $ratio_orig) {
  $width = $height*$ratio_orig;
} else {
  $height = $width/$ratio_orig;
}
// Resample
$image_p = imagecreatetruecolor($thumbsize, $thumbsize);
$image = imagecreatefromjpeg($filename);
imagecopyresampled($image_p, $image, -($width/2) + ($thumbsize/2), -($height/2) + ($thumbsize/2), 0, 0, $width, $height, $width_orig, $height_orig);
// Output
imagejpeg($image_p, null, 100);
?>

Here's a simple function to resample one JPEG imagefile to another while keeping aspect ratio of the source within the destination's dimensions. You can also tune the allowable distortion if you end up making too many thumbnails with thin blank areas around them. Should work when enlarging images too. Function returns true if it worked, false if not.
function resample_picfile($src, $dst, $w, $h)
{
  // If distortion stretching is within the range below,
  // then let image be distorted.
  $lowend = 0.8;
  $highend = 1.25;
  $src_img = imagecreatefromjpeg($src);
  if($src_img)
  {
    $dst_img = ImageCreateTrueColor($w, $h);
    /* if you don't want aspect-preserved images 
to have a black bkgnd, fill $dst_img with the color of your choice here. 
    */
    if($dst_img)
    {
      $src_w = imageSX($src_img);
      $src_h = imageSY($src_img);
      $scaleX = (float)$w / $src_w;
      $scaleY = (float)$h / $src_h;
      $scale = min($scaleX, $scaleY);
      $dstW = $w;
      $dstH = $h;
      $dstX = $dstY = 0;
      $scaleR = $scaleX / $scaleY;
      if($scaleR < $lowend || $scaleR > $highend)
      {
        $dstW = (int)($scale * $src_w + 0.5);
        $dstH = (int)($scale * $src_h + 0.5);
        // Keep pic centered in frame.
        $dstX = (int)(0.5 * ($w - $dstW));
        $dstY = (int)(0.5 * ($h - $dstH));
      }
      
      imagecopyresampled(
        $dst_img, $src_img, $dstX, $dstY, 0, 0, 
        $dstW, $dstH, $src_w, $src_h); 
      imagejpeg($dst_img, $dst); 
      imagedestroy($dst_img);
    }
    imagedestroy($src_img);
    return file_exists($dst);
  }
  return false;
}

Here is a function I thought I would share that will resample and copy an image with rounded corners.
<?php
/** ------------------------------------------------------------
 * Copy and resample an image with rounded corners.
 * ----------------------------------------------------------- */
function imageRoundedCopyResampled(&$dstimg, &$srcimg, $dstx, $dsty, $srcx,
                  $srcy, $dstw, $dsth, $srcw, $srch, $radius) {
  # Resize the Source Image
  $srcResized = imagecreatetruecolor($dstw, $dsth);
  imagecopyresampled($srcResized, $srcimg, 0, 0, $srcx, $srcy,
            $dstw, $dsth, $srcw, $srch);
  # Copy the Body without corners
  imagecopy($dstimg, $srcResized, $dstx+$radius, $dsty,
       $radius, 0, $dstw-($radius*2), $dsth);
  imagecopy($dstimg, $srcResized, $dstx, $dsty+$radius,
       0, $radius, $dstw, $dsth-($radius*2));
  # Create a list of iterations; array(array(X1, X2, CenterX, CenterY), ...)
  # Iterations in order are: Top-Left, Top-Right, Bottom-Left, Bottom-Right
  $iterations = array(
    array(0, 0, $radius, $radius),
    array($dstw-$radius, 0, $dstw-$radius, $radius),
    array(0, $dsth-$radius, $radius, $dsth-$radius),
    array($dstw-$radius, $dsth-$radius, $dstw-$radius, $dsth-$radius)
  );
  # Loop through each corner 'iteration'
  foreach($iterations as $iteration) {
    list($x1,$y1,$cx,$cy) = $iteration;
    for ($y=$y1; $y<=$y1+$radius; $y++) {
      for ($x=$x1; $x<=$x1+$radius; $x++) {
        # If length (X,Y)->(CX,CY) is less then radius draw the point
        $length = sqrt(pow(($cx - $x), 2) + pow(($cy - $y), 2));
        if ($length < $radius) {
          imagecopy($dstimg, $srcResized, $x+$dstx, $y+$dsty,
               $x, $y, 1, 1);
        }
      }
    }
  }
}
?>

Another addition to zorroswordsman at gmail dot com's resize class. 
The function takes $_FILES['sent_image'] as 1st parameter. The 2nd is complete destination path. 
It only moves images. 
It returns destination path if succeeded, and false if any error occurred. 
<?php
public function move_image($tmp_img, $dest_img)
{
  //verifies if the uploaded file is an image
  if (strpos($tmp_img['type'], 'image') !== false) 
    {
    //moves the uploaded file into the destination place
    if (move_uploaded_file($tmp_img['tmp_name'], $dest_img)) {
      return $dest_img;
    }
  }    
  return false;
}
?>

Another tiny ammendment to zorroswordsman at gmail dot com's resize class AND matt at rees-jenkins dot co dot uk addition. The class may resize to different width and height not only same ones:
<?php
// Set maximum image size (pixels)
public function set_size($max_x = 100,$max_y = 100)
{
  
  // Resize
  if($this->x_input > $max_x || $this->y_input > $max_y)
  {
    $a= $max_x / $max_y;
    $b= $this->x_input / $this->y_input;
    
    if ($a<$b)
    {
      $this->x_output = $max_x;
      $this->y_output = ($max_x / $this->x_input) * $this->y_input;
    }
    else
    {
      $this->y_output = $max_y;
      $this->x_output = ($max_y / $this->y_input) * $this->x_input;
    }
    // Ready
    
    $this->resize = TRUE;
    
  }
  
  // Don't resize    
  else { $this->resize = FALSE; }
  
}
?>
And the use of the class is now:
<?php
##### DEMO #####
// Image
$src = "myimage.jpg";
// Begin
$img = new imaging;
$img->set_img($src);
$img->set_quality(80);
// Small thumbnail
$img->set_size(250,150);
$img->save_img("small_250x150_" . $src);
// Baby thumbnail
$img->set_size(50,250);
$img->save_img("baby_50x250_" . $src);
// Finalize
$img->clear_cache();
?>

None of the stuff I've seen for resizing transparent GIFs works consistently and yields a good image. The hack I thought of is silly, but it works okay -- convert GIF to PNG. This worked for me.
$g_iw is new image width
$g_ih is new image height
$img_src=imagecreatefromgif($g_srcfile);
$img_dst=imagecreatetruecolor($g_iw,$g_ih);
//preserve alpha
imagecolortransparent($img_dst, imagecolorallocate($img_dst, 0, 0, 0));
imagealphablending($img_dst, false);
imagesavealpha($img_dst, true);
imagecopyresampled($img_dst, $img_src, 0, 0, 0, 0, $g_iw, $g_ih, $g_is[0], $g_is[1]);
imagepng($img_dst, $g_dstfile); 
imagedestroy($img_dst);

This function is taken from lots of web sources so thanks to all for posting. It creates square or landscape thumbnails from .jpgs from either portait or landscape original images. I decide in advance which way I want the thumbs to display for consistency. I usually sharpen the images as well post-upload, to save on server resources. I can post this code if anyone wants it. I hope this helps someone...
  # create thumbnails from jpgs
  # usage:
  # create_jpgthumb(uploaded file, final file (with path), thumb height, thumb width, jpg quality, scale thumb (true) or fixed size (false); 
  function create_jpgthumb($original, $thumbnail, $max_width, $max_height, $quality, $scale = true) {
    
    list ($src_width, $src_height, $type, $w) = getimagesize($original);
    
    if (!$srcImage = @imagecreatefromjpeg($original)) {
      return false;
    }
    # image resizes to natural height and width
    if ($scale == true) {
            
      if ($src_width > $src_height ) {
        $thumb_width = $max_width;
        $thumb_height = floor($src_height * ($max_width / $src_width));
      } else if ($src_width < $src_height ) {
        $thumb_height = $max_height;
        $thumb_width = floor($src_width * ($max_height / $src_height));
      } else {
        $thumb_width = $max_height;
        $thumb_height = $max_height;
      }
      if (!@$destImage = imagecreatetruecolor($thumb_width, $thumb_height)) {
        return false;
      }
      
      if (!@imagecopyresampled($destImage, $srcImage, 0, 0, 0, 0, $thumb_width, $thumb_height, $src_width, $src_height)) {
        return false;
      }
            
    # image is fixed to supplied width and height and cropped
    } else if ($scale == false) {
    
      $ratio = $max_width / $max_height;
        
      # thumbnail is landscape
       if ($ratio > 1) {
       
        # uploaded pic is landscape
        if ($src_width > $src_height) {
          $thumb_width = $max_width;
          $thumb_height = ceil($max_width * ($src_height / $src_width));
          
          if ($thumb_height > $max_width) {
            $thumb_height = $max_width;
            $thumb_width = ceil($max_width * ($src_width / $src_height));
          }
        # uploaded pic is portrait
        } else {
        
          $thumb_height = $max_width;
          $thumb_width = ceil($max_width * ($src_height / $src_width));
          
          if ($thumb_width > $max_width) {
            $thumb_width = $max_width;
            $thumb_height = ceil($max_width * ($src_height / $src_width));
          }
          
          $off_h = ($src_height - $src_width) / 2;
      
        }
  
        if (!@$destImage = imagecreatetruecolor($max_width, $max_height)) {
          return false;
        }
    
        if (!@imagecopyresampled($destImage, $srcImage, 0, 0, 0, $off_h, $thumb_width, $thumb_height, $src_width, $src_height)) {
          return false;
        }
      # thumbnail is square
       } else {
       
        if ($src_width > $src_height) {
          $off_w = ($src_width - $src_height) / 2;
          $off_h = 0;
          $src_width = $src_height;
        } else if ($src_height > $src_width) {
          $off_w = 0;
          $off_h = ($src_height - $src_width) / 2;
          $src_height = $src_width;
        } else {
          $off_w = 0;
          $off_h = 0;
        }
        if (!@$destImage = imagecreatetruecolor($max_width, $max_height)) {
          return false;
        }
    
        if (!@imagecopyresampled($destImage, $srcImage, 0, 0, $off_w, $off_h, $max_width, $max_height, $src_width, $src_height)) {
          return false;
        }
       }
     
              
    }
    
    @imagedestroy($srcImage);
    if (!@imageantialias($destImage, true)) {
      return false;
    }
    
    if (!@imagejpeg($destImage, $thumbnail, $quality)) {
      return false;
    }
            
    @imagedestroy($destImage);
    
    return true;
  }

imagecopyresampled() works amazingly well, but here is something NOT to try with it:
The resize routine I wrote looks for pixel widths, and resizes images based on the notion that if their widths and heights are too big, so are their byte sizes. It worked out well at first.
But when I saw huge byte-size images slipping through beneath my 1000px-wide benchmark, I figured I'd subject the smaller-width images to resizing if their filesize() exceeded 100K.
The idea was that I'd "resize" them keeping their original widths and heights (i.e. in (1) below, $newW = $oldW, $newH = $oldH), and letting the "75" at (2) below reduce the byte size.
DON'T try that. imagecopyresampled() will lock up the server trying to resize an 800px-wide image to a "new" width of 800px. (I caught that on my Windows server before ever putting it on the destination Linux server, so take that with a grain of salt.)
I got around it by making $newW = ($picW * 0.99), etc. You get the byte-size reduction you want without locking up.
(1) imagecopyresampled($image_p, $image, 0, 0, 0, 0, $newW, $newH, $picW, $picH);
(2) imagejpeg($image_p, $theFile, 75);

Tim's function is a whole lot faster, however, the quality setting doesnt seem right since resizing very big images doesnt affect quality of the resulting thumbnails that much on quality 1 for example.
Anyone has figured out how to use a more correct quality setting by comparing image surfaces (for example) basing on Tim's function?
images with a total of 10Megapixels that contain 5Megapixels when resized should use the same quality setting like images that contain 1Megapixels but only 0.5 when resized.
Not hard to understand, but the memoryload would be a lot lower if the function could decide to use a setting of quality 1 automatically when resizing big images to small thumbnails. This would be handy especially when using the function for images of various sizes in the same application.

Tim's code is fast, but if you're trying to put resampled images anywhere but in the top-left corner, it doesn't work.

The suggestion of converting a gif to a png to retain transparency was useful. However the steps used to maintain the transparency work for a gif as well, so the conversion step is not necessary. Just do everything the same, and save as a gif anyway. i.e.
<?php
$g_iw is new image width
$g_ih is new image height
$img_src=imagecreatefromgif($g_srcfile);
$img_dst=imagecreatetruecolor($g_iw,$g_ih);
//preserve alpha
imagecolortransparent($img_dst, imagecolorallocate($img_dst, 0, 0, 0));
imagealphablending($img_dst, false);
imagesavealpha($img_dst, true);
imagecopyresampled($img_dst, $img_src, 0, 0, 0, 0, $g_iw, $g_ih, $g_is[0], $g_is[1]);
imagegif($img_dst, $g_dstfile); 
imagedestroy($img_dst); 
?>

Here's a little function I wrote to resize images to a maximum dimension - based on what facebook does in the galleries. You put in a source, destination and a maximum dimension in pixels (eg 300), and for example if the image is long and thin, the longest edge will be 300px, yet the image retains proportions. A square image will become 300x300, a 6x4 (landscape) will become 300x200, a 4x6 (portrait) - 200x300 etc.
It works on jpg images, but other formats can easily be added.
<?php
function createThumb($spath, $dpath, $maxd) {
 $src=@imagecreatefromjpeg($spath);
 if (!$src) {return false;} else {
 $srcw=imagesx($src);
 $srch=imagesy($src);
 if ($srcw<$srch) {$height=$maxd;$width=floor($srcw*$height/$srch);}
 else {$width=$maxd;$height=floor($srch*$width/$srcw);}
 if ($width>$srcw && $height>$srch) {$width=$srcw;$height=$srch;} //if image is actually smaller than you want, leave small (remove this line to resize anyway)
 $thumb=imagecreatetruecolor($width, $height);
 if ($height<100) {imagecopyresized($thumb, $src, 0, 0, 0, 0, $width, $height, imagesx($src), imagesy($src));}
 else {imagecopyresampled($thumb, $src, 0, 0, 0, 0, $width, $height, imagesx($src), imagesy($src));}
 imagejpeg($thumb, $dpath);
 return true;
 }
}
?>

Be wary of this function when resizing images to make them *larger* than the original due to the memory consumption rate. For example a 200KB JPEG file (1024x768) will take up 4MB of memory when loaded, but when resampled to twice the the size the memory use jumps to 20.1MB. imagecopyresized does the same. Allow approx. 5 bytes per *pixel* for memory allowance when dealing with true colour images.