PHP random password generator

   function generate_password($len = 9)
   {

        $characters = '123456789ABCDEFGHIJKLMNOPQRSTUVWXZ';

        $string = '';    

        for ($p = 0; $p < $len; $p++) {

            $string .= $characters[mt_rand(0, strlen($characters))];    
        }

        return $string;

    }