Fix PHP @ linking function ($15)

I've got 1 function that links @ symbols like Twitter. A excellent coder could probably whiz this in half hour to and hour. I'm gonna post the function in the window, because I don't want to get a newb that can't help. Read the function, if you know the solution, you can fix for money. Bugs: when and unknown name, that isn't in my DB is first it won't link even the known names, but it will if the known name comes first. - if the DB connection fails, you’ll get an "cannot call method prepare() on null" error - preg_replace() does work, but you never save the modified string but return the original string. and you’ll also have a problem for similar usernames like "bob1" and "bob12" (the former tag also matches the latter) I need this to nail the tagging dead on, as in any professional app like twitter. I know my code is sloppy, but hope you can fix this. I'll require it working before money is release, so I'll want to test the function. ----- public function handle_user_link($str){ if( preg_match_all('/([@]+\w+)/', $str, $matches)){ $matches = array_slice($matches, 0, 1); for($i = 0; $i < count($matches); $i++) { // capture each tag $tag = $matches[0][$i]; $name = ltrim($tag, '@'); try{ $db = new PDO($this->db['dsn'], $this->db['username'], $this->db['password'], array(PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION)); }catch(\PDOException $e){} $sqlName = $db->prepare("SELECT `username` FROM `users` WHERE `username` = ?"); $sqlName->execute(array($name)); $retrievedUser = $sqlName->fetch(); if(!empty($retrievedUser)) { // tagged user exists. $str = str_replace($tag, '<a href="[login to view URL]'.$name.'">' . $tag . '</a>' , $str); } } } return $str; // End handle_user_link }