On the Label Media blog today there's a new post from Tom Rawcliffe (the first part of a series) about a common design pattern you can use in your applications - the factory pattern.
A design pattern is a template used by software engineers as a guideline of how to solve a particular problem. [...] In this series of articles I plan to explore some of the design patterns that i have found most useful in my time as a PHP developer, providing examples and notes. So, this being the first, we’ll take a look at the Factory Method Pattern.
He defines the pattern - an interface to a class without the caller having to know what type of class it is - and a code example showing an abstract class with a "Factory" method that pulls information from either a jpeg or png class based on the extension of the file name given.