Lars Tesmer and his coworkers have been working on improving their development skills lately, specifically related to unit testing. In his latest post he shares some of what they've discovered along the way.

The plan was to try and write as many tests as we could for the Constraint classes PHPUnit uses to implement its assertions. [...] Well, our plan didn’t work out that way, we didn’t really succeed in writing a considerable amount of unit tests. However, it still was a valuable experience, as it turned out the unit tests of the Constraints are a good example of how not to unit test.

He includes three of the major points they came across in their practice development:

• Don’t use one single test case class to test several different classes
• Name your tests well
• Avoid to test more than one behaviour in one single test

For each, there's a summary answering the "why" question behind them including an example test (testConstraintIsType) that shows a bad, multiple assertion practice that should be avoided if possible.