A promise represents a single result of an asynchronous operation. It is not necessarily available at a specific time, but should become in the future.

The PHP-HTTP promise follows the Promises/A+ standard.


Work is underway for a Promise PSR. When that PSR has been released, we will use it in HTTPlug and deprecate our Http\Promise\Promise interface.

Asynchronous requests

Asynchronous requests enable non-blocking HTTP operations. When sending asynchronous HTTP requests, a promise is returned. The promise acts as a proxy for the response or error result, which is not yet known.

To execute such a request with HTTPlug:

$request = $messageFactory->createRequest('GET', '');

// Where $client implements HttpAsyncClient
$promise = $client->sendAsyncRequest($request);

// This code will be executed right after the request is sent, but before
// the response is returned.
echo 'Wow, non-blocking!';

See HTTP Factories on how to use message factories.

Although the promise itself is not restricted to resolve a specific result type, in HTTP context it resolves a PSR-7 Psr\Http\Message\ResponseInterface or fails with an Http\Client\Exception.


An asynchronous request will never throw an exception directly but always return a promise. All exceptions SHOULD implement Http\Client\Exception. See Exceptions for more information on the exceptions you might encounter.


The $promise that is returned implements Http\Promise\Promise. At this point in time, the response is not known yet. You can be polite and wait for that response to arrive:

try {
    $response = $promise->wait();
} catch (\Exception $exception) {
    echo $exception->getMessage();


Instead of waiting, however, you can handle things asynchronously. Call the then method with two arguments: one callback that will be executed if the request turns out to be successful and/or a second callback that will be executed if the request results in an error:

    // The success callback
    function (ResponseInterface $response) {
        echo 'Yay, we have a shiny new response!';

        // Write status code to some log file
        file_put_contents('responses.log', $response->getStatusCode() . "\n", FILE_APPEND);

        return $response;

    // The failure callback
    function (\Exception $exception) {
        echo 'Oh darn, we have a problem';

        throw $exception;