In another of his series looking "under the covers" at what actually happens in the PHP language during its use, Sharif Ramadan has posted this look at the object handling in PHP's OOP functionality.
I would love to take a good long look under the hood at just how PHP objects and classes do the work that they do, and hope that you could benefit from that knowledge. [There are] many questions that come across my desk, on a regular basis, from developers and beginner PHP enthusiasts that I’ve worked with over the years, and are some of the key points this article attempts to help you answer.
He talks about classes "giving birth" to objects, how they're stored internal to PHP and how they provide the "blueprints" for it to lay out the storage of the object's data. He talks about using identifiers for variable/property access, object handlers and how "$this" fits into all of it. He notes that OOP, while a major part of PHP now, wasn't in the initial versions (until around PHP4). He finishes off the post talking about lateral/vertical context switching, the lifecycle of an object and the "early binding problem" and class scope.