As a part of his "Advancing PHP Developer" series, Eran Galperin has posted part five, a look at design patterns and what they can do for you and your application.
A design pattern is a general reusable solution to a recurring design problem in object-oriented systems. Design patterns are essentially blueprints that suggest how to solve a particular set of OO design problems while adhering to OO best good-practices (which I've recounted in my Object Oriented part of this series).
He talks about one of the most popular examples - the Model/View/Controller design pattern that is used in many rapid development frameworks for PHP (including CodeIgniter and the Zend Framework). He also briefly mentions a few others like the composite, singleton and decorator patterns.