A controller reaches into middleware to run business logic. It looks convenient. The middleware has the session, the user, the request body, everything you need. So why bounce through a handler when you can just validate the cart right there? Because the moment you do, your business rule only runs inside an HTTP pipeline, and you just lost the ability to invoke it from anywhere else.
Open a typical PHP project. You see Controllers/, Services/, Repositories/, Entities/. You know it uses Symfony or Laravel. You don't know if it manages a library, processes invoices, or tracks shipments. The structure tells you about the framework, not the business.
Most PHP developers start with MVC. You know where things go: controllers handle requests, models handle data, views handle output. Then the project grows, and the model becomes 800 lines of ORM queries, validation rules, and business logic fighting for space.
Most PHP frameworks are installed as Composer packages. They end up in vendor/, behind a version constraint, and you interact with them through their public API. You don't read the implementation. You don't modify it. When something breaks after an update, you adapt your code to match the new version.
You have been doing MVC for ten years. Someone drops "Clean Architecture" in a meeting. You nod.
But concretely, what becomes of your Model?