public static function encrypt($string, $key) {
        $result = '';
        for ($i=0; $i<strlen ($string); $i++) {
            $char = substr($string, $i, 1);
            $keychar = substr($key, ($i % strlen($key))-1, 1);
            $char = chr(ord($char)+ord($keychar));
            $result .= $char;
        }

        return base64_encode($result);
    }