While doing research for an upcoming talk about php middlewares, I came across these blog posts and libraries that I thought were really insightful and interesting.

Laravel Middleware Documentation

The talk itself was for a Laravel meetup and Laravel as always has really good documentation that makes the concepts easy to understand.

http interop — http Middleware

This is the readme for the http interopt working groups middleware implementation. It more filled out than the fig's read me. It has a lot of good information on different middleware implementations.

ircmaxell — All About Middleware

This blog post was published shortly after the FIG Http Middleware spec was proposed. It explains why the author believes the original spec contained poor design decisions and gives working examples.

RelayPHP

A framework agnostic middleware stack. The readme on this stack also contains a lot of good informations and thoughts about middleware design and interface. It's worth reading especially in-conjunction with Ircmaxell's blog post.

Weierophinney's Talk

These are slides from Weierophinney's talk about PSR 7 and middlewares. It contains a good history and theoretical explanations of middlewares.