In my experience, the best place to look for text on object-oriented programming is beginners' C++ books. Sure, many of the features described there aren't available in PHP, but the ones that are are usually presented in a very slow, very easy to grasp manner with copious amounts of examples to make sure you get the point. My favourite is "Teach yourself C++ in 21 Days" by Jesse Liberty - this has so much content on what OOP is and why it is helpful that nearly all readers can benefit.
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