Payum\Paypal\ExpressCheckout\Nvp\Api::doRequest PHP Method

    protected function doRequest(array $fields)
    {
        $headers = array('Content-Type' => 'application/x-www-form-urlencoded');
        $request = $this->messageFactory->createRequest('POST', $this->getApiEndpoint(), $headers, http_build_query($fields));
        $response = $this->client->send($request);
        if (false == ($response->getStatusCode() >= 200 && $response->getStatusCode() < 300)) {
            throw HttpException::factory($request, $response);
        }
        $result = array();
        parse_str($response->getBody()->getContents(), $result);
        foreach ($result as &$value) {
            $value = urldecode($value);
        }
        return $result;
    }