private function generateInitialHash()
    {
        $mergedString = $this->isLive . $this->transaction['orderId'] . $this->transaction['invoiceNumber'] .
            $this->transaction['amount'] . $this->customer['telephoneNumber'] . $this->customer['email'] .
            $this->vendorId . $this->currency . $this->payloads['payload1'] . $this->payloads['payload2'] .
            $this->payloads['payload3'] . $this->payloads['payload4'] . $this->callback .
            $this->customer['sendEmail'] . $this->callbackMode;

        return hash_hmac('sha1', $mergedString, $this->hashKey);
    }