Adding on a bit more to a previous post of his look at test isolation (ex. global variables from one test do not effect any others') with an update he's made to the PHPUnit code concerning the isolation.
Since the previous posting, I have added a backup/restore mechanism for static attributes of classes to PHPUnit. This is yet another feature of PHPUnit that makes the testing of code that uses global state (which includes, but is not limited to, global and superglobal variables as well as static attributes of classes) easier.
Two graphs illustrate the difference - one showing a normal run and another with this new feature in use and showing off the performance increase it can give.