On PHPMaster.com there's a new tutorial from Lorna Mitchell about some of the lesser known OOP features that are built in to the PHP language. She talks about things like interface inheritance, private properties and autoloading and type hints.
The vast majority of today's applications written in PHP are object-oriented, and in general the core OOP concepts are pretty well understood by PHP developers. This article pushes the boundary of your understanding and shows you some tricks, or potential pitfalls depending on your perspective, of OOP in PHP.
Besides the ones mentioned above, she also looks at abstract classes and their use as well as the use of "finally" to handle the cleanup after exceptions.