PHPFreaks has posted part one and part two of a new series they're working on to introduce those new to the language to the concepts of object-oriented programming in PHP.
On the old site, I started a series of tutorials named "PHP Application Design". I planned for at least three parts, and while I did publish the first two and wrote at least half of part three, I found myself out of time to make it to the finish. Still dedicated to writing reusable and extensible PHP code and spreading the knowledge, this is a restart of the series, renamed as simply "OO PHP"
In part one he looks at the basics of OOP (terms, defining properties/methods, autoload, exceptions, type hinting, etc) with some code examples sprinkled through out. Part two looks at the "boring OOP principles" like polymorphism, encapsulation and heuristics.