Mike Bernat has started off a new series looking at design patterns and how they can be used in PHP applications.
From his introductory post:
Over the next couple weeks I'm going to be writing a series on programming design patterns and their applications. I plan to cover the popular patterns as well as the lesser-known, yet still insanely useful, ones too.
The introduction goes through the basics of what design patterns are, how they're used and segways into the next article - a look at the singleton.