DevShed continues their look at using polymorphism in an application with the latest part of the series - "Building Dynamic Web Pages with Polymorphism in PHP 5".
In short, Polymorphism is a feature exposed by certain objects that belong to the same family, which eventually can behave differently, even when they're using identical methods. Or more clearly, an object can be considered polymorphic when it's capable of performing different actions by utilizing the same method.
This time they focus on web page development that uses this object-oriented practice. The create a WebPageElement that you can ID and class attributes on and use it to create HTML widgets and extend them to create Div and Link element.