I\'ve been writing PHP for about six years now and have got to a point where I feel I should be doing more to write better code. I know that Object Oriented code is the way
Instead of learning OO from scratch, I think it'd be easier if you took on a framework that facilitates object-oriented programming. It will "force" you to use the right OOP methods; you will be able to learn from the way the framework is written as to how to do OOP best.
I'd recommend the QCodo PHP5 framework http://www.qcodo.com. It has great video tutorials on how to set it up, as well as video trainings (http://www.qcodo.com/demos/).
Full disclosure: I've been developing on top of this framework for two years, and I've contributed code to their codebase (so I'm not completely impartial :-)).