In this recent post Brandon Savage suggests that just using objects in your application doesn't mean that it's truly "object oriented development." There's other criteria that need to be met to really fit this description.

Lots of developers understand that object oriented code offers advantages over procedural programming. And so, they begin working on creating objects in their own projects, and eventually feel pretty good about what they’ve done. After all, if they’re using objects, their code must be object oriented, right? Well, not exactly.

He breaks it down into three main points that developers should consider when working with OOP in their apps: splitting responsibilities between classes, being polymorphic and using dependency injection. There's no code samples to back up the concepts here, but it's a decent list to think about. There's plenty of tutorials out there about SOLID development and dependency injection in PHP apps, so you might check some of those out to help with these concepts.