Overview

The Slim Framework implements a version of the Rack protocol. As a result, a Slim application can have middlewarethat may inspect, analyze, or modify the application environment, request, and response before and/or after theSlim application is invoked.

Middleware Architecture

Think of a Slim application as the core of an onion. Each layer of the onion is middleware. When you invoke theSlim application’s run() method, the outer-most middleware layer is invoked first. When ready, that middlewarelayer is responsible for optionally invoking the next middleware layer that it surrounds. This process steps deeperinto the onion - through each middleware layer - until the core Slim application is invoked. This stepped processis possible because each middleware layer, and the Slim application itself, all implement a public call() method.When you add new middleware to a Slim application, the added middleware will become a new outer layer and surroundthe previous outer middleware layer (if available) or the Slim application itself.

Application Reference

The purpose of middleware is to inspect, analyze, or modify the application environment, request, and responsebefore and/or after the Slim application is invoked. It is easy for each middleware to obtain references to theprimary Slim application, its environment, its request, and its response:

<?php
class MyMiddleware extends \Slim\Middleware
{
    public function call()
    {
        //The Slim application
        $app = $this->app;
        //The Environment object
        $env = $app->environment;
        //The Request object
        $req = $app->request;
        //The Response object
        $res = $app->response;
    }
}

Changes made to the environment, request, and response objects will propagate immediately throughout the applicationand its other middleware layers. This is possible because every middleware layer is given a reference to the sameSlim application object.

Next Middleware Reference

Each middleware layer also has a reference to the next inner middleware layer with $this->next. It is eachmiddleware’s responsibility to optionally call the next middleware. Doing so will allow the Slim applicationto complete its full lifecycle. If a middleware layer chooses not to call the next inner middleware layer,further inner middleware and the Slim application itself will not be run, and the application response willbe returned to the HTTP client as is.

<?php
class MyMiddleware extends \Slim\Middleware
{
    public function call()
    {
        //Optionally call the next middleware
        $this->next->call();
    }
}