private function getRequestHeaders($url, array $parameters)
    {
        $headers = array();

        if (isset($this->clientId) && isset($this->sharedSecret)) {
            $objectIdentifier = null;
            if ($this->apiVersion === '1.1' && isset($parameters['id'])) {
                $objectIdentifier = $parameters['id'];
            } elseif ($this->apiVersion === '1.0' && isset($parameters['api_id'])) {
                $objectIdentifier = $parameters['api_id'];
            }

            $headers['X-Tulip-Client-ID'] = $this->clientId;
            $headers['X-Tulip-Client-Authentication'] = hash_hmac('sha256', $this->clientId.$url.$objectIdentifier, $this->sharedSecret);
        }

        return $headers;
    }