تولید رشته تصادفی در PHP

<?php
    /**
     * Generates a random string of a given length and character set.
     * @param int $l The length of the resulting string
     * @param string $c The character set
     * @return string The random sequence
     */
    function mt_rand_str ($l, $c = 'abcdefghijklmnopqrstuvwxyz1234567890') {
    for ($s = '', $cl = mb_strlen($c)-1, $i = 0; $i < $l; $s .= mb_substr($c, mt_rand(0, $cl), 1), ++$i);
    return $s;
    }
    ?>