lib/Client/MultiCurl.php
public function sendRequest(RequestInterface $request, array $options = []): ResponseInterface
    {
        $options = $this->validateOptions($options);
        $originalCallback = $options->get('callback');
        $responseToReturn = null;
        $options = $options->add(['callback' => function (RequestInterface $request, ResponseInterface $response = null, ClientException $e = null) use (&$responseToReturn, $originalCallback) {
            $responseToReturn = $response;
            $originalCallback($request, $response, $e);

            if (null !== $e) {
                throw $e;
            }
        }]);

        $this->addToQueue($request, $options);
        $this->flush();

        return $responseToReturn;
    }
public function sendRequest(RequestInterface $request, array $options = []): ResponseInterface
    {
        $options = $this->validateOptions($options);
        $originalCallback = $options->get('callback');
        $responseToReturn = null;
        $options = $options->add(['callback' => function (RequestInterface $request, ResponseInterface $response = null, ClientException $e = null) use (&$responseToReturn, $originalCallback) {
            $responseToReturn = $response;
            $originalCallback($request, $response, $e);

            if (null !== $e) {
                throw $e;
            }
        }]);

        $this->addToQueue($request, $options);
        $this->flush();

        return $responseToReturn;
    }
This is a blocking function call.