First, make sure you understand some of the higher level OO concepts of PHP like variable scope, inheritance, polymorphism, and visibility. You can easily do this in the php docs and stackoverflow, or other such Google searches.
Then take a look at Codeigniter, http://codeigniter.com/user_guide/ it's organized, explains the how and why of MVC, is very very well documented and has a lot of great libraries for getting started, then once you get familiar with how everything works you can begin to look under the hood and start figuring out why it works the way it does and continue learning from there.