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

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);
}
?>

double quotes !== single quotes

php > echo nl2br('\r\n');
\r\n
php > echo nl2br("\r\n");
<br />

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);
}
?>