After 10 years of coding in PHP both OSS and products, creating 2 frameworks used in production on numerous websites, I still wonder. Why do you need to stuff interfaces in the mix? Why such complex patterns for something so simple?

What I do and it works very well, is that I register plugins, callbacks in the application configuration:

     $cfg = ['plugins_for_foo' => ['callback_one', 'callback_two', 
                                  [$Obj, 'three']]];

Then in the code I get my configuration like this:

     $callbacks = Conf::get('plugins_for_foo', array());

And I can iterate over them and activate them. It is fast, flexible, you can easily test the callbacks in your unit tests, etc. Note that this is the standard approach to load components with Django.