All of the above poster have really good points. I would highly recommend the following books:
The OO Thought Process: http://www.amazon.com/Object-Oriented-Thought-Process-3rd/dp/0672330164/ref=sr_1_5?ie=UTF8&s=books&qid=1262547120&sr=8-5
PHP5 Objects, Patterns & Practice: http://www.amazon.com/PHP-Objects-Patterns-Practice-Second/dp/1590599098/ref=sr_1_3?ie=UTF8&s=books&qid=1262546817&sr=8-3
OO Design Heuristics: http://www.amazon.com/Object-Oriented-Design-Heuristics-Arthur-Riel/dp/020163385X/ref=sr_1_1?ie=UTF8&s=books&qid=1262546869&sr=8-1
I read earlier edition of "Object-Oriented Thought Process" and found the book immensely helpful in understanding the whole OOP paradigm.