17. HTTP messages are the foundation of web
development. Web browsers and HTTP
clients such as cURL create HTTP request messages
that are sent to a web server,
which provides an HTTP response message.
Server-side code receives an HTTP
request message, and returns an HTTP response
message.
30. The terminology "pipeline" is often used to describe
the onion. One way of looking at the "onion" is as a
queue, which is first-in-first-out (FIFO) in operation.
This means that the first middleware on the queue is
executed first, and this invokes the next, and so on
(and hence the "next" terminology).
31. return function (Application $app, MiddlewareFactory $factory, ContainerInterface
$container) : void {
$app->pipe(ErrorHandler::class);
$app->pipe(ServerUrlMiddleware::class);
$app->pipe(RouteMiddleware::class);
$app->pipe(ImplicitHeadMiddleware::class);
$app->pipe(ImplicitOptionsMiddleware::class);
$app->pipe(MethodNotAllowedMiddleware::class);
// Seed the UrlHelper with the routing results:
$app->pipe(UrlHelperMiddleware::class);
$app->pipe(DispatchMiddleware::class);
$app->pipe(NotFoundHandler::class);
}