I saw some code from this website:
return preg_replace('@((https?://)?([-\w]+\.[-\w\.]+)+\w(:\d+)?(/([-\w/_\.]*(\?\S+)?)?)*)@', '<a href="$1">$1</a>', $text);
$1 if used for
preg_replace ($pattern , $replacement , $subject)
replacement may contain references of the form \n or (since PHP 4.0.4) $n, with the latter form being the preferred one. Every such reference will be replaced by the text captured by the n'th parenthesized pattern. n can be from 0 to 99, and \0 or $0 refers to the text matched by the whole pattern.
from php doc: http://docs.php.net/manual/en/function.preg-replace.php