In a new post to DZone.com Giorgio Sironi talks about the "open/closed principle" in software development and shows an example based on the design of the PHPUnit_Selenium project.

This article shows an example of how the application of the Open/Closed Principle improved the design of a real project, the open source library PHPUnit_Selenium. These design concepts apply to every object-oriented language, including Java, Ruby or even C++. The Open Closed Principle, part of SOLID set, states that software should be open for extension and at the same time closed for modification.

He starts with a little background on the project, pointing out that there's a Session object it uses for all of its testing with a magic "__call" method that handles any kind of method call to the object. This method has issues (dependencies, strict requirements for use) but can be refactored according to the Open/Closed idea to set up an array of anonymous functions that can be called as a "command". Examples of these types of classes are also included (one for the "click" action on a button and another for getting the current location).