public static function random($len = 6)
    {
        $chars  = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
        $random = '';
        for ($p = 0; $p < $len; $p++) {
            $random .= $chars[mt_rand(0, 61)];
        }

        return $random;
    }