Tom Rawcliffe has posted the latest in his design patterns series covering some of the most common patterns and how they would work in PHP. In this new post he looks at the observer pattern.

So far in my series of articles on design patterns in PHP we've looked at a creational pattern, a structural pattern and a behavioral pattern. Today I'll be taking a closer look at another behavioral pattern - the observer. The observer pattern (also known as the Subscribe/Publish Pattern) is used to notify one object, the observer, about a change of state from another object, the subject.

He gives the layout of the pattern in a simple UML diagram before moving into the description and code. His sample builds a Users object that has a set of observers on it. His custom observer is a logger that, when a change is made on the Users, it writes out a value of the current object.