current()

(PHP 4, PHP 5, PHP 7)

返回数组中的当前单元

说明

current(array &$array): mixed

每个数组中都有一个内部的指针指向它“当前的”单元，初始指向插入到数组中的第一个单元。

参数

$array

这个数组。

返回值

current()函数返回当前被内部指针指向的数组单元的值，并不移动指针。如果内部指针指向超出了单元列表的末端，current()返回FALSE。

Warning

此函数可能返回布尔值FALSE，但也可能返回等同于FALSE的非布尔值。请阅读布尔类型章节以获取更多信息。应使用===运算符来测试此函数的返回值。

范例

使用current()系列函数的例子

<?php
$transport = array('foot', 'bike', 'car', 'plane');
$mode = current($transport); // $mode = 'foot';
$mode = next($transport);    // $mode = 'bike';
$mode = current($transport); // $mode = 'bike';
$mode = prev($transport);    // $mode = 'foot';
$mode = end($transport);     // $mode = 'plane';
$mode = current($transport); // $mode = 'plane';
$arr = array();
var_dump(current($arr)); // bool(false)
$arr = array(array());
var_dump(current($arr)); // array(0) { }
?>

注释

Note:如果数组包含booleanFALSE的单元则本函数在碰到这个单元时也返回FALSE，使得不可能判断是否到了此数组列表的末端。要正确遍历可能含有空单元的数组，用each()函数。

参见

• end() 将数组的内部指针指向最后一个单元
• key() 从关联数组中取得键名
• each() 返回数组中当前的键／值对并将数组指针向前移动一步
• prev() 将数组的内部指针倒回一位
• reset() 将数组的内部指针指向第一个单元
• next() 将数组中的内部指针向前移动一位

current() also works on objects:
<?php
 echo current((object) array('one', 'two')); // Outputs: one
?>

Note, that you can pass array by expression, not only by reference (as described in doc).
<?php
var_dump( current( array(1,2,3) ) ); // (int) 1
?>

If you do current() after using uset() on foreach statement, you can get FALSE in PHP version 5.2.4 and above.
There is example:
<?php
$prices = array(
  0 => '1300990',
  1 => '500',
  2 => '600'
);
foreach($prices as $key => $price){
  if($price < 1000){
    unset($prices[$key]);
  }
}
var_dump(current($prices)); // bool(false)
?>
If you do unset() without foreach? all will be fine.
<?php
$prices = array(
  0 => '1300990',
  1 => '500',
  2 => '600'
);
unset($prices[1]);
unset($prices[2]);
var_dump(current($prices));
?>

Array functions, such as `current()` and `rewind()` will work on `Traversable` as well, PHP 5.0 - 7.3, but not in HHVM:
<?php
$queue = new ArrayIterator(array('adasdasd'));
reset($queue);
$current = current($queue);
var_dump($current);
?>
See https://3v4l.org/VjCHR

For large array(my sample was 80000+ elements), if you want to traverse the array in sequence, using array index $a[$i] could be very inefficient(very slow). I had to switch to use current($a).

To that "note": You won't be able to distinguish the end of an array from a boolean FALSE element, BUT you can distinguish the end from a NULL value of the key() function.
Example:
<?php
if (key($array) === null) {
  echo "You are in the end of the array.";
} else {
  echo "Current element: " . current($array);
}
?>

Note that by copying an array its internal pointer is lost:
<?php
$myarray = array(0=>'a', 1=>'b', 2=>'c');
next($myarray);
print_r(current($myarray));
echo '<br>';
$a = $myarray;
print_r(current($a));
?>
Would output 'b' and then 'a' since the internal pointer wasn't copied. You can cope with that problem using references instead, like that:
<?php
$a =& $myarray;
?>

The docs do not specify this, but adding to the array using the brackets syntax:
   <?php $my_array[] = $new_value; ?>
will not advance the internal pointer of the array. therefore, you cannot use current() to get the last value added or key() to get the key of the most recently added element.
You should do an end($my_array) to advance the internal pointer to the end ( as stated in one of the notes on end() ), then
  <?php
   $last_key = key($my_array); // will return the key
   $last_value = current($my_array); // will return the value
  ?>
If you have no need in the key, $last_value = end($my_array) will also do the job.
- Sergey.

Array can be passed by both REFERENCE and EXPRESSION on `current`, because current doesn't move array's internal pointer,
this is not true for other functions like: `end`, `next`, `prev` etc.
<?php
  function foo() {return array(1,2,3);}
  echo current(foo()); // this print '1'
  echo end(foo());   // this print error: Only variables should be passed by reference
?>

Based on this example http://php.net/manual/en/function.current.php#116128 i would like to add the following. As Vasily points out in his example 
<?php
$prices = array(
  0 => '1300990',
  1 => '500',
  2 => '600'
);
foreach($prices as $key => $price){
  if($price < 1000){
    unset($prices[$key]);
  }
}
var_dump(current($prices)); // bool(false)
?>
The above example will not work and return false for version of PHP between 5.2.4 and 5.6.29. The issue is not present on PHP versions >= 7.0.1
A different workaround (at least from Vasily's example) would be to use reset() before using current() in order to reset the array pointer to start.
<?php
$prices = array(
  0 => '1300990',
  1 => '500',
  2 => '600'
);
foreach($prices as $key => $price){
  if($price < 1000){
    unset($prices[$key]);
  }
}
reset($prices);
var_dump(current($prices)); // string(7) "1300990"
?>

It took me a while to figure this out, but there is a more consistent way to figure out whether you really went past the end of the array, than using each().
You see, each() gets the value BEFORE advancing the pointer, and next() gets the value AFTER advancing the pointer. When you are implementing the Iterator interface, therefore, it's a real pain in the behind to use each().
And thus, I give you the solution:
To see if you've blown past the end of the array, use key($array) and see if it returns NULL. If it does, you're past the end of the array -- keys can't be null in arrays.
Nifty, huh? Here's how I implemented the Iterator interface in one of my classes:
<?php
/**
 * DbRow file
 * @package PalDb
 */
/**
 * This class lets you use Db rows and object-relational mapping functionality.
 */
 
class DbRow implements Iterator
{
  /**
   * The DbResult object that gave us this row through fetchDbRows
   * @var DbResult
   */
  protected $result;
  
  /**
   * The fields of the row
   * @var $fields
   */
  protected $fields;
    
  /**
   * Constructor
   *
   * @param PDOStatement $stmt
   * The PDO statement object that this result uses
   * @param DbResult $result
   * The result that produced this row through fetchDbRows 
   */
  function __construct($result)
  {
    $this->result = $result;
  }
  
  /**
   * Get the DbResult object that gave us this row through fetchDbRows
   * @return DbResult
   *
   * @return unknown
   */
  function getResult()
  {
    return $this->result;
  }
  
  function __set(
    $name, 
    $value)
  {
    $this->fields[$name] = $value;
  }
  
  function __get(
    $name)
  {
    if (isset($this->fields[$name])) 
      return $this->fields[$name];
    else
      return null;
  }
  
  /**
   * Iterator implementation - rewind
   */
  function rewind()
  {
    $this->beyondLastField = false;
    return reset($this->fields);
  }
  
  function valid()
  {
    return !$this->beyondLastField;
  }
  
  function current()
  {
    return current($this->fields);
  }
  
  function key()
  {
    return key($this->fields);
  }
  
  function next()
  {
    $next = next($this->fields);
    $key = key($this->fields);      
    if (isset($key)) {
      return $next[1];
    } else {
      $this->beyondLastField = true;
      return false; // doesn't matter what we return here, see valid()
    }
  }
  
  private $beyondLastField = false;
};
Hope this helps someone.

A simple copy function that not only copies the given array but ensures the copy's pointer is set to the exact same position:
<?php
function array_copy(&array)
{
  $key = key($array);
  $copy = $array;
  while (($copy_key = key($copy)) !== NULL) {
    if ($copy_key == $key) break;
    next($copy);
  }
  return $copy;
}
?>
That's all ... bye.

If we unset any element from an array, and then try the current function, I noted it returned FALSE. To overcome this limitation, you can use array_values function to re-order the tree.

As Sergey stated a long time ago in this notes, when adding to the array using the brackets syntax, current won't return the last added value. Instead of moving the pointer with end(), you simply can get the last value with <?php $my_array[count($my_array) - 1] ?> (only applies to a non-associative array).