A simple PHP function to strip strings and numbers.

strip_strings.php

<?php
function strip_string($str, $case) {
    switch ($case) {
        case "az": return preg_replace("/[^a-zA-Z]+/", "", $str);
        case "num": return preg_replace("/[^0-9]+/", "", $str);
        case "num+az": default: return preg_replace("/[^a-zA-Z0-9]+/", "", $str);
    }
}
?>