PHP create transaction ID - random string

<?php /** * Make a random string, maybe use to transaction ID * @param @length int length of result * @param string $chars characters use for random * @return string Random string */ function _airlineTicket($length=9, $chars=NULL) { if (NULL == $chars) { // makes a random alpha numeric string of a given lenth $chars = array_merge(range('A', 'Z'), range('a', 'z'),range(0, 9)); } $out =''; for($c=0;$c < $length;$c++) { $out .= $chars[mt_rand(0,count($chars)-1)]; } return strtoupper($out); }