<?php
$string = "wordey; string, boom";
$string = preg_replace("/(.*?)([^a-z]+)/i",
"<a href='x'>$1</a>$2", $string);
$string = preg_replace("/, (.*)/", ", <a href='x'>$1</a>", $string);
echo $string; // <a href='wordey'>wordey</a>; <a href='string'>string</a>, <a href='boom'>boom</a>
?>