protected function buildGetCreditPayload()
    {
        $xml = new \SimpleXMLElement(
            '<?xml version="1.0" encoding="ISO-8859-1"?>'.
            '<!DOCTYPE REQUESTCREDIT SYSTEM "http://127.0.0.1:80/psms/dtd/requestcredit.dtd">'.
            '<REQUESTCREDIT></REQUESTCREDIT>'
        );

        $xml->addAttribute('USERNAME', $this->username);
        $xml->addAttribute('PASSWORD', $this->password);

        return $xml->asXml();
    }