On DZone.com today there's a new tutorial from Giorgia Sironi giving an example of working with the PageObject pattern and how it's implemented to help with testing in the PHPUnit_Selenium plugin.
In the realm of acceptance testing, a possibility for hooking into the application and exercising it end-to-end is to work through a user interface, which nowadays is in more and more cases web-based. The Page Object pattern is a way to model pages (intended as screens) of a web application as independent objects, and give them several responsibilities.
The pattern sets up the resources to let you make queries against the page's contents, interacting with elements on the page, running common assertions on the contents and moving around on the site. He gives a more practical example of it in action (again, using PHPUnit_Selenium) to test a login page and evaluate a successful login.