nl2br()

(PHP 4, PHP 5, PHP 7)

在字符串所有新行之前插入 HTML 换行标记

说明

nl2br(string $string[,bool $is_xhtml= TRUE] ): string

在字符串$string所有新行之前插入'<br />'或'<br>'，并返回。

参数

$string: 输入字符串。
$is_xhtml: 是否使用 XHTML 兼容换行符。

返回值

返回调整后的字符串。

范例

Example #1nl2br()使用范例

<?php
echo nl2br("foo isn't\n bar");
?>

以上例程会输出：

foo isn't<br />
 bar

使用$is_xhtml生成合法的 HTML 标记

<?php
echo nl2br("Welcome\r\nThis is my HTML document", false);
?>

以上例程会输出：

Welcome<br>
This is my HTML document

各种换行分隔符

<?php
$string = "This\r\nis\n\ra\nstring\r";
echo nl2br($string);
?>

以上例程会输出：

This<br />
is<br />
a<br />
string<br />

更新日志

版本	说明
5.3.0	新增可选的$is_xhtml参数。在此之前，总是插入'<br />'。

参见

htmlspecialchars()将特殊字符转换为 HTML 实体
htmlentities()将字符转换为 HTML 转义字符
wordwrap()打断字符串为指定数量的字串
str_replace()子字符串替换

It's important to remember that this function does NOT replace newlines with <br> tags. Rather, it inserts a <br> tag before each newline, but it still preserves the newlines themselves! This caused problems for me regarding a function I was writing -- I forgot the newlines were still being preserved.
If you don't want newlines, do:
<?php
$Result = str_replace( "\n", '<br />', $Text );
?>

to replace all linebreaks to <br />
the best solution (IMO) is:
<?php
function nl2br2($string) {
$string = str_replace(array("\r\n", "\r", "\n"), "<br />", $string);
return $string;
}
?>
because each OS have different ASCII chars for linebreak:
windows = \r\n
unix = \n
mac = \r
works perfect for me

double quotes !== single quotes
php > echo nl2br('\r\n');
\r\n
php > echo nl2br("\r\n");
<br />

If you write code that is to run in browser AND on the shell, this function could be useful. It uses PHP_EOL or <br /> depending on the platform it runs:
<?php
function nl($string) {
 if(isset($_SERVER['SHELL'])) return preg_replace('/\<br(\s*)?\/?\>/i', PHP_EOL, $string);
 return nl2br($string);
}
print nl("One\nTwo<br>Three\r\nFour<br />Five
Six" . PHP_EOL);
?>

Here's a more simple one:
<?php
/**
 * Convert BR tags to nl
 *
 * @param string The string to convert
 * @return string The converted string
 */
function br2nl($string)
{
  return preg_replace('/\<br(\s*)?\/?\>/i', "\n", $string);
}
?>
Enjoy

Starting from PHP 4.3.10 and PHP 5.0.2, this should be the most correct way to replace <br /> and <br> tags with newlines and carriage returns.
<?php
/**
 * Convert BR tags to newlines and carriage returns.
 *
 * @param string The string to convert
 * @return string The converted string
 */
function br2nl ( $string )
{
  return preg_replace('/\<br(\s*)?\/?\>/i', PHP_EOL, $string);
}
?>
(Please note this is a minor edit of this function: http://php.net/nl2br#86678 )
You might also want to be "platform specific", and therefore this function might be of some help:
<?php
/**
 * Convert BR tags to newlines and carriage returns.
 *
 * @param string The string to convert
 * @param string The string to use as line separator
 * @return string The converted string
 */
function br2nl ( $string, $separator = PHP_EOL )
{
  $separator = in_array($separator, array("\n", "\r", "\r\n", "\n\r", chr(30), chr(155), PHP_EOL)) ? $separator : PHP_EOL; // Checks if provided $separator is valid.
  return preg_replace('/\<br(\s*)?\/?\>/i', $separator, $string);
}
?>

Seeing all these suggestions on a br2nl function, I can also see that neither would work with a sloppy written html line break.. Users can't be trusted to write good code, we know that, and mixing case isn't too uncommon.
I think this little snippet would do most tricks, both XHTML style and HTML, even mixed case like <Br> <bR /> and even <br      > or <br   />.
<?php
function br2nl($text)
{
  return preg_replace('/<br\\s*?\/??>/i', '', $text);
}
?>

This is example with "\R" regex token which matches any unicode newline character.
"u" flag treate strings as UTF-16. Which is optional, depending on your use case.
<?php
public function nl2br($string)
{
 return preg_replace('/\R/u', '<br/>', $string);
}
?>
NOTE:
preg_replace versions are much slower than using str_replace version or built-in nl2br. 
Check out pcre.backtrack_limit php.ini setting for information about PCRE limit. It's good to know.
Some PHP7 benchmarks:
<?php 
// nl2br()
function nl2br_str($string) {
 return str_replace(["\r\n", "\r", "\n"], '<br/>', $string);
}
function nl2br_preg_R($string)
{
 return preg_replace('/\R/u', '<br/>', $string);
}
function nl2br_preg_rnnr($string)
{
 return preg_replace('/(\r\n | \n | \r)/', '<br/>', $string);
}
?>
 # nl2br
 ## Time: 0.02895712852478 s
 # nl2br_str
 ## Time: 0.027923107147217 s
 # nl2br_preg_R
 ## Time: 0.13350105285645 s
 # nl2br_preg_rnnr
 ## Time: 0.14213299751282 s

I test empirically this function nl2br and nl2br2 (create by ngkongs at gmail dot com).
Both work nice with different ASCII chars for linebreak, but the function nl2br2 is faster than nl2br.
nl2br2 ~ 0.0000309944153 s
nl2br ~ 0.0011141300201 s
The function nl2br2:
<?php 
function nl2br2($string) { 
 $string = str_replace(array("\r\n", "\r", "\n"), "<br />", $string); 
 return $string; 
} 
?>

Let's say a form text field does contain:
 test line number 1
 test line number 2
To remove all the line breaks and split the input into an array:
$input_from_text_field = preg_replace('#[\r\n]#','',nl2br($input_from_text_field,false));
$lines = explode("<br>",$input_from_text_field);
$lines will now be an array where $lines[0] = "test line number 1" and so on.

After a recent post at the forums on Dev Shed, I noticed that it isn't mentioned, so I will mention it.
nl2br returns pure HTML, so it should be after PHP anti-HTML functions ( such as strip_tags and htmlspecialchars ).

I just spent a whole day trying to figure out why my textarea $_POST was not getting <br /> tags added by nl2br(). So for others
<?php filter_input(INPUT_POST, 'text', FILTER_SANITIZE_SPECIAL_CHARS); ?> returns newlines as &#13; and &#10; so nl2br will miss them.
rather use <?php filter_input(INPUT_POST, 'text', FILTER_SANITIZE_STRING); ?> and newlines will remain intact so nl2br will pick them up.

for bbcode :
<?php
$message = nl2br(preg_replace('#(\\]{1})(\\s?)\\n#Usi', ']', stripslashes($message)));
?>

This one works with br tags having attributes, in any case,
closed or not closed, and does not double linefeeds
<?php
 /**
 * convert br tags to nl
 *
 * @param string The string to convert
 * @return string The converted string
 */
function br2nl($string)
{
  return preg_replace("/<br[^>]*>\s*\r*\n*/is", "\n", $string);
}
?>
I combine this with strip_tags() for dead simple "contenteditable" fields allowing only text and linefeeds.

On the contrary, <b>mark at dreamjunky.comno-spam</b>, this function is rightfully named. Allow me to explain. Although it does re-add the line break, it does so in an attempt to stay standards-compliant with the W3C recommendations for code format.
According to said recommendations, a new line character must follow a line break tag. In this situation, the new line is not removed, but a break tag is added for proper browser display where a paragraph isn't necessary or wanted.