On april 22th 2013 I delivered a free lecture about PHP. The lecture took place in HIT. You can download the slides I was using during the lecture. You can find (within the slides) the links we mention during the lecture. For a smooth start I recommend on starting with learning the basics www.phpbook.co.il.
The Visitor Design Pattern
The visitor design pattern allows us to add operations to objects that already exist without modifying their classes and without extending them.