public function calculateMd5Signature()
    {
        return strtoupper(md5(
            $this->getMerchantId() .
            $this->getTransactionReference() .
            $this->getSecretWordForMd5Signature() .
            $this->getSkrillAmount(true) .
            $this->getSkrillCurrency() .
            $this->getStatus()
        ));
    }