private function getShortUrlId($len = 7) {
        $chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890';
        $charsLen = 62;
        $key = '';

        for ($i = 0; $i < $len; $i++) {
            $key .= $chars[mt_rand() % $charsLen];
        }

        return $key;
    }