public static function randStr($len = 6, $format = 'ALL')
    {
        switch (strtoupper($format)) {
            case 'ALL':
                $chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-@#~';
                break;
            case 'CHAR':
                $chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz-@#~';
                break;
            case 'NUMBER':
                $chars = '0123456789';
                break;
            default :
                $chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-@#~';
                break;
        }
        mt_srand((double)microtime() * 1000000 * getmypid());
        $randStr = "";
        while (strlen($randStr) < $len)
            $randStr .= substr($chars, (mt_rand() % strlen($chars)), 1);
        return $randStr;
    }