private function shopSignature(
        $amount,
        $order,
        $merchantCode,
        $currency,
        $transactionType,
        $merchantURL,
        $secret
    ) {
        return strtoupper(sha1(implode('', [
            $amount,
            $order,
            $merchantCode,
            $currency,
            $transactionType,
            $merchantURL,
            $secret,
        ])));
    }