lib/Client/MultiCurl.php
public function sendAsyncRequest(RequestInterface $request, array $options = []): void
    {
        $options = $this->validateOptions($options);

        $this->addToQueue($request, $options);
    }
public function sendAsyncRequest(RequestInterface $request, array $options = []): void
    {
        $options = $this->validateOptions($options);

        $this->addToQueue($request, $options);
    }
Populates the supplied response with the response for the supplied request.

If a "callback" option is supplied, its value will be called when the
request completes. It is ONLY in the callback you will see the response
or an exception.

This is a non-blocking function call.

The callable should have the following signature:

$callback = function($request, $response, $exception) {
if (!$exception) {
// success
} else {
// error ($error is one of the CURLE_* constants)
}
};