On the Blue Parabola blog, Matthew Turland takes a look at object-oriented programming and what core concepts lie at its heart.

What is object-oriented programming? The acronym OOP has become a bit of a buzzword in the current age of programming, to the point where the waters of its definition have become rather murky. [...] PHP may not be object-oriented, but from a purist perspective, neither is Java. What do I mean by "purist perspective?" Plain and simple: everything is either an object or a message being passed between objects (where message parameters are also objects).

He mentions one of the first languages to support objects (Simula) and the four fundamental concepts that would make a language truly OOP - abstraction, inheritance, encapsulation, polymorphism. Its his opinion, though, that while its good for languages to adhere to these four principles as much as they can, discussions about how well they adhere to them is usually just "spinning your wheels" and don't have much use.