Paul Jones has a new post to his blog looking at two approaches to building applications in PHP - either using an include-based of class-based architecture, briefly looking at the advantages and disadvantages of each.
This is a topic I describe at length in my Organizing [Your PHP Projects] talk, but I would like to give the idea a home on my blog for easy reference.
He talks about the typical structure of a PHP application - at least where most start out - being the "include" method. He talks about the progression it usually makes into functions and classes and then, eventually, a jump in concept to the "class" method similar to what several popular frameworks have gone with.
The difference is that no program logic is executed at loading time with a class file. Loading the class file makes it available for use, but the programmer has to instantiate it and then call methods on it for anything else to happen.