Anthony Ferrara has written up a post for his site sharing more information about middleware and the PSR-7 proposal that's being discussed to help standardize interfaces with this popular form of application processing.

Last week, a proposal to standardize middleware for PSR-7 was introduced to the PHP-FIG. The general concept of middleware is not a new one, and has been in use in the PHP world for many years. Several people have raised significant concerns with the proposal, which have gone completely unheeded by the author. Let me go through the most major of these concerns, as well as show what a better proposal might look like.

He starts off with a brief look at the current proposal - the interface it defines and an example of a real world usage of it to check some attributes on the request/response. He gives a few more examples before getting into what he sees as the fundamental problem with the interface: that it passes in a response instance rather than creating its own ("what does $response mean inside the middleware?"). He's mostly talking about context and not knowing from one middleware to the next what kind of changes may have been made to the response. He also includes some of the arguments on the "for" side of including the parameter and an interesting list of middleware that does this modification prior to the next() call, making it difficult to determine the actual state.

He ends the post with a few other issue he has with the proposal including the use of the __invoke method name, a restriction on typing and the next method being callable. He makes a few suggestions of modifications to the proposal that he thinks could help make it better, correcting these issues.