The WACT (Web Application Component Toolkit) wiki on PHP application design concerns:

"PHP does not enforce the notion that everything is an object. Successful PHP applications mix both procedural and object oriented logic. Developers from Java background, for example, often begin by writing a class "Object" then extended it with all classes. That's already overhead you'll likely regret. It's important to use PHP in the "sense" it was designed to be used, which is a mix of both paradigms. Related to MVC, this may mean you implement the Model (and below) as classes while the Controller and View are procedural."