What is middleware?

Middleware is code that exists between the request and response, and which can take the incoming request, perform actions based on it, and either complete the response or pass delegation on to the next middleware in the queue.

// In public/index.php:
use Zend\Diactoros\Response;
use Zend\Diactoros\Server;
use Zend\Stratigility\Middleware\NotFoundHandler;
use Zend\Stratigility\MiddlewarePipe;

use function Zend\Stratigility\middleware;
use function Zend\Stratigility\path;

require __DIR__ . '/../vendor/autoload.php';

$app = new MiddlewarePipe();

$server = Server::createServer([$app, 'handle'], $_SERVER, $_GET, $_POST, $_COOKIE, $_FILES);

// Landing page
$app->pipe(middleware(function ($req, $handler) {
    if (! in_array($req->getUri()->getPath(), ['/', ''], true)) {
        return $handler->handle($req);

    $response = new Response();
    $response->getBody()->write('Hello world!');

    return $response;

// Another page
$app->pipe(path('/foo', middleware(function ($req, $handler) {
    $response = new Response();

    return $response;

// 404 handler
$app->pipe(new NotFoundHandler(function () {
    return new Response();

$server->listen(function ($req, $res) {
    return $res;

In the above example, we have two examples of middleware. The first is a landing page, and listens at the root path. If the request path is empty or /, it completes the response. If it is not, it delegates to the next middleware in the stack. The second middleware matches on the path /foo — meaning it will match /foo, /foo/, and any path beneath. In that case, it will complete the response with its own message. If no paths match at this point, a "final handler" is composed by default to report 404 status.

So, concisely put, middleware are PHP callables that accept a request object, and do something with it, optionally delegating creation of a response to another handler.

PSR-15 middleware

Stratigility supports only PSR-15 middleware.

Middleware can decide more processing can be performed by calling the $handler instance passed during invocation. With this paradigm, you can build a workflow engine for handling requests — for instance, you could have middleware perform the following:

  • Handle authentication details
  • Perform content negotiation
  • Perform HTTP negotiation
  • Route the path to a more appropriate, specific handler

Each middleware can itself be middleware.

Using the provided PathMiddlewareDecorator (created by the path() function demonstrated in the initial example), you can also attach middleware to specific paths, allowing you to mix and match applications under a common domain. As an example, you could put API middleware next to middleware that serves its documentation, next to middleware that serves files, and segregate each by URI:

$app->pipe(path('/api', $apiMiddleware));
$app->pipe(path('/docs', $apiDocMiddleware));
$app->pipe(path('/files', $filesMiddleware));

The handlers in each middleware attached this way will see a URI with that path segment stripped, allowing them to be developed separately and re-used under any path you wish.

Within Stratigility, middleware must be PSR-15 middleware. Zend\Stratigility\MiddlewarePipe implements Psr\Http\Server\MiddlewareInterface.

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