A quick and easy way to ensure your tests don’t have any unwanted side-effects is to randomise the order in which they run. PHPUnit makes this easy.
Alternatively, you can pass the
--order-by=random flag to the command-line test runner.
Try this with any sizeable test suite, and there’s a decent chance you’ll see a few unexpected failures.
Sign up for my newsletter
A monthly round-up of blog posts, projects, and internet oddments.