Slim is a microframework that offers routing capabilities for easily creating small PHP applications. But an interesting, and powerful, feature is its concept of Middleware. [...] I've found middleware to be an eloquent solution for implementing various filter-like services in a Slim app, such as authentication and caching. In this article I'll explain how middleware works and share with you a simple cache example that highlights how you can implement your own custom middleware.

He talks about what "middleware" is (complete with illustration) and how Slim can be used as a layer in the middleware stack. His example is a caching layer, based on Slim, that takes a request, checks the cache for it and returns it if it exists. If not, it saves the content to a database. He also includes code examples of how to use the "add" method to introduce your middleware libraries into the Slim application.

If you'd like more examples, the Slim project has several middleware examples provided in their "extras" github repository.