Different frameworks use middleware differently. Slim adds middleware as concentric layers surrounding your core application. Each new middleware layer surrounds any existing middleware layers. The concentric structure expands outwardly as additional middleware layers are added.
The last middleware layer added is the first to be executed.
When you run the Slim application, the Request object traverses the middleware structure from the outside in. They first enter the outer-most middleware, then the next outer-most middleware, (and so on), until they ultimately arrive at the Slim application itself. After the Slim application dispatches the appropriate route, the resultant Response object exits the Slim application and traverses the middleware structure from the inside out. Ultimately, a final Response object exits the outer-most middleware, is serialized into a raw HTTP response, and is returned to the HTTP client. Here’s a diagram that illustrates the middleware process flow :
