I think I did it on my own. I looked at how to use preg_replace, because I never have before..
PHP Code:
<?php
$pattern = array('/@([A-Za-z0-9]*)/');
$replace = array('@<a href="http://twitter.com/$1">$1</a>');
$subject = "@mrheadrick i think i figured it out on my own!";
echo preg_replace($pattern, $replace, $subject, 1);
?>