public static function makeRandomString( $length, $char_set = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_' )
    {
        $ret = '';
        $char_set_cnt = strlen($char_set);

        mt_srand();
        for($i = 0; $i < $length; $i++){
            $idx = mt_rand(0, $char_set_cnt - 1);
            $ret .= $char_set[ $idx ];
        }

        return $ret;
    }