Making the transition from the procedural PHP world into the object-oriented world of some more complex applications can be difficult. It requires a shift in thinking that might take some learning. Lorna Mitchell has posted an introduction to some of the common concepts.

Since the introduction of PHP 5 in 2004, PHP has had an object model worthy of that description and became a truly modern language for use on the web. [...] Using OOP (Object Orientated Programming) enables us to architect our systems much more clearly, and to make them more manageable and more maintainable. This technique also allows us to separate form from function to create clean, navigable codebases with plenty of opportunities to reuse code, apply design patterns and bring in concepts from other brances of computer science.

She covers some of the more common pieces you'll need to know to start venturing into OOP in PHP including: objects versus classes, using properties/methods, inheritance and access modifiers. Some sample code is included that works with user records in a database (along with the SQL to create the tables).