In this new tutorial on PHPMaster.com Jose Asuncion walks you through the use of mock objects in your unit testing.

If you are part of a development team, more often than not your code will also depend on code written by your teammates. But what if their code isn’t available at the moment - say, your teammate hasn’t finished writing it yet? Or, what if the code you need requires other external dependencies that are hard to set up? And what if you cannot test code because of other factors beyond your control? Will you just hang around, do nothing, and wait until your team is done or when everything is in place? Of course not! In this this article, I’ll illustrate how to write code that works around this problem with dependencies.

He starts off by making a case for using mock objects and having them replace things that either aren't finished or depends on things that are hard to replicate (like external dependencies). He includes some example code showing them in use in a PHPUnit-based test and how to use its own mocking abilities so you don't have to manually make mock classes yourself.