protected function setHttpClientConfiguration()
    {
        $this->setCurlConstants();

        $this->httpClientConfig = [
            CURLOPT_SSLVERSION     => CURL_SSLVERSION_TLSv1_2,
            CURLOPT_SSL_VERIFYPEER => $this->validateSSL,
        ];

        if (!empty($this->certificate)) {
            $this->httpClientConfig[CURLOPT_SSLCERT] = $this->certificate;
        }

        // Initialize Http Client
        $this->setClient();

        // Set default values.
        $this->setDefaultValues();

        // Set PayPal API Endpoint.
        $this->apiUrl = $this->config['api_url'];

        // Set PayPal IPN Notification URL
        $this->notifyUrl = $this->config['notify_url'];
    }