Giorgio Sironi has just wrapped up his posts in a series on practical testing of PHP applications with PHPUnit in nine parts:
What is unit testing and why a php programmer should adopt it? It may seem simple, but testing is the only way to ensure your work is completed and you will not called in the middle of the night by a client whose website is going nuts. [...] Unit testing is the process of writing tests which exercise the basic functionality of the smallest cohesive unit in php: a class.
Here's a list of the topics covered in the series: