In this chapter, you learned about Unit Testing and why it is a subtantial improvement over less formal component-testing methodologies. You learned the principles of any unit-testing framework and met one of the more popular frameworks in the form of PEAR's PHPUnit. You saw how to install it and the basics behind how it works. You then applied it to a useful real-world debugging example to track down an errant bug in a class, thereby proving just how useful it really is.
In the next chapter, you'll meet an incredibly useful concept from the earlier years of computer science that of the Finite State Machine and how it can be resurrected in PHP 5 to help solve some of the trickier algorithmic problems you might encounter in application development.