The first thing that you must understand about OOP is that it presents not just new syntax but a new way of thinking about a problem. By far the most common mistake beginning OOP programmers make is to inappropriately apply OOP theory. PHP will tell you when you make a syntactical mistake, but you’ll need to learn how to avoid the oretical mistakes as well. To explain...

All programming comes down to taking actions with data: a user enters data in an HTML form; the PHP code validates it, emails it, and stores it in a database; and so forth. These are simply verbs (actions) and nouns (data). With procedural programming, the focus is on the verbs: do this, then this, then this. In OOP, the focus is on the nouns: with what types of things will the application work? In both approaches, you need to identify both the nouns and the verbs required; the difference is in the focus of the application’s design.