I noticed a lot of sites allow user submitted content but prevent autolinking of urls.  Some of the most common instances of this are on <a href="http://twitter.com">Twitter</a> sites (much like my own creation: <a title="SecretTweet - anonymous Twitter" href="http://secrettweet.com">SecretTweet.com</a>).  So I threw together a function to autolink urls:
<?
$text= 'Hey go to google.com moo http://google.com moo www.google.com moo http://wwww.google.com/404.php moo';
function linkify(&$text)
{
//CONVERT *://.* TO LINK (EG HTTP://KEVINSMITHDESIGNS.COM)
$text = ereg_replace("[a-zA-Z]+://([.]?[a-zA-Z0-9_/-])*", "\\0", $text);
//CONVERT *://*.* TO LINK (EG HTTP://WWW.KEVINSMITHDESIGNS.COM)
$text = ereg_replace("(^| )(www([.]?[a-zA-Z0-9_/-])*)", "\\1\\2", $text);
return $text;
}
echo linkify($text);
?>

Would return "Hey go to google.com  moo <a href="http://google.com">http://google.com</a> moo <a href="http://google.com">www.google.com</a> moo <a href="http://wwww.google.com/404.php">http://wwww.google.com/404.php</a> moo".

And since I made this primarily for <a href="http://twitter.com">Twitter</a>, the following will linke @ replies automatically, too:
<?
$tweet = "Hey @mozunk! Go check out http://digsby.com -- I think you will like it.";
function twitterify(&$tweet) {
//CONVERT *://.* TO LINK (EG HTTP://KEVINSMITHDESIGNS.COM)
$tweet = ereg_replace("[a-zA-Z]+://([.]?[a-zA-Z0-9_/-])*", "\\0", $tweet);
//CONVERT *://*.* TO LINK (EG HTTP://WWW.KEVINSMITHDESIGNS.COM)
$tweet = ereg_replace("(^| )(www([.]?[a-zA-Z0-9_/-])*)", "\\1\\2", $tweet);
//LINK @USERNAME
$tweet = preg_replace('/([\.|\,|\:|\¡|\¿|\>|\{|\(]?)@{1}(\w*)([\.|\,|\:|\!|\?|\>|\}|\)]?)\s/i', "$1@$2$3 ", $tweet);return $tweet;
}
echo twitterify(&$tweet);
?>

Would return "Hey <a href="http://twitter.com/mozunk">@mozunk</a>!  Go check out <a href="http://digsby.com">http://digsby.com</a> -- I think you will like it."

I'll be implementing this into <a href="http://secrettweet.com">SecretTweet</a> pretty soon.  I haven't decided if I want to autolink URLs or not...maybe just @replies.
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automatically link urls php
autolink @ replies Twitter automatically link php