Matthew Weier O'Phinney, lead developer on the Zend Framework project, has written up a new post for his site showing how to use anonymous classes to write middleware for your applications (several frameworks support the concept of middleware these days).
I faced an interesting question recently with regards to middleware: What happens when we go from a convention-based to a contract-based approach when programming?
Convention-based approaches usually allow for duck-typing; with middleware, it means you can write PHP callables - usually closures - and just expect them to work.
Contract-based approaches use interfaces. I think you can see where this is going.
He starts off looking at some of the currently popular methods for creating middleware basic them off of either the PSR-7 standard or the proposed PSR-15 HTTP middleware, each with code examples to illustrate. The post then gets to the point with a look at anonymous class-based middleware and some of the advantages they provide. He refactors both a PSR-7 middleware and a closure-based middleware over to PSR-15 using this method.