Phil Sturgeon has a post over on his site sharing some of his thoughts on PHP middleware and why he thinks it's worth paying attention to in your applications.
Recently there has been a lot of buzz about HTTP middleware in PHP. Since PSR-7 was accepted, everyone and their friend Sherly has been knocking out middleware implementations, some of them stunning, some of them half-arsed, and some of them rolled into existing frameworks. HTTP Middleware is a wonderful thing, but the PHP-FIG is working on a specific standard for middleware, which will standardise this mess of implementations, but some folks don't seem to think that would be useful.
Let's look into middleware a little closer, to show you why it's something to smile about.
He starts with a bit of background about the history of middleware in the PHP ecosystem and where they fit in the overall execution path. He lists out some of the middlewares that have already come out based on this surge in the community including CSRF protection, debugging and rate limiting handling. With various frameworks handling the request/response slightly differently, the PHP-FIG worked up a standard to make interoperability easier. He links to some other resources about middleware that have been posted and discussions he's had with other people about their usefulness.
HTTP Middleware is awesome. It lets frameworks do far less, it lets people distribute logic in a way often unseen popularly in PHP, it lets more of your application be reusable, and it lets PHP catch up with other popular languages used to build stuff on the web. PSR-7 was a great step towards this goal, but we need another PSR to get the whole way there.