Does anyone know of a good tutorial or book that teaches the concept of OOP in PHP? I've looked around this site and many others, but I can't find anything that I can completely understand.
Whenever you're looking for tutorials, always check out the PHP ResourceIndex. http://php.resourceindex.com/Documentation/Class_Design_and_OOP/ has a lot of information about OOP.
.. if you're looking for outdated tutorials, yes.
One of the best sites about OOP and PHP is http://www.phppatterns.com/
do you have the php manual ?