In this new tutorial posted to DevShed, they take a look at working with classes in PHP5, specifically using the keywords for "member visibility" to make your classes a bit more structured.

The previous concept can also be applied successfully when it comes to defining the visibility of properties and methods in PHP classes. [...] Using the jargon of object-oriented programming, this capacity is widely known as member visibility. This capacity allows PHP programmers to establish whether the set of methods and properties of a specific class will be public, protected, or private.

They start more generally by looking at how to set up the variables globally and then move down the line talking about restrictions (private/protected/public) and how to get to them in the scripts outside the class.