On the Coders' View blog there's a recent post looking at using the strategy design pattern in your PHP application.
In the strategy pattern, algorithms are extracted from complex classes so they can be replaced easily. For example, the strategy pattern is an option if you want to change the way pages are ranked in a search engine. Think about a search engine in several parts - one that iterates through the pages, one that ranks each page, and another that orders the results based on the rank.
Their example shows a class that, based on a few different search "strategies", can locate user information in the user listing. Their example uses a random and a "find after" strategy and includes the results.