Here are some resources if you want to read more about the concept. The original set of exercises made for Java is in the Thoughtworks Anthology, in a chapter by Jeff Bay, over the years after porting this to PHP I have decided to gradually revert back to original exercise strictness as I find we have evolved to a point where we can be strict. This means some of the older material will have a flowing set of guidelines.
- The Thoughtworks Anthology (http://l.doh.ms/1iqkrvB)
- The Art of Readable Code (http://l.doh.ms/1lje5U8)