The PHP Freaks website has a new tutorial posted today thats an introduction to design patterns - what they are and which were the ideas of the "Gang of Four".
Implementing Design Patterns is gradually getting more common in the PHP world. The hype around Ruby on Rails, which is based on the Model-View-Controller architectural pattern, has spawned a generation of PHP based frameworks which embrace this pattern also, paving the way for others to embrace design patterns in general in their PHP applications.
Design patterns are just a standardized way of doing something (like a Factory where, when you request an object, if one already exists you get that one instead of a new one). They illustrate the design pattern concept with a database abstraction system (UML) and a configuration setup where the main class inherits from the child type-specific ones.