Generate a random code

<?php
    public function createUserCode(){
        $code = '' ;
        $chars = "abcdefghijkmnopqrstuvwxyz023456789"; 
        srand((double)microtime()*1000000); 
        for ($i=0;$i<=8;$i++) { 
        $num = rand()%20; 
        $tmp = substr($chars, $num, 1); 
        $code = $code.$tmp; 
        }
    return $code; 
    }
?>