Debugging non-deterministic tests in PHPUnit

You have a test that fails when run as part of your test suite. You re-run the test on its own, and it passes. What gives? More than likely you have a non-deterministic test, and probably a test that lacks isolation.

Here’s a tip for debugging tests that are breaking isolation: a PHPUnit Listener can check which of your test cases is changing the environment and causing a failure of a later test case. For example, suppose your tests rely on the APP_ENV environment variable being set to test. The following PHPUnit listener will check this before every test:

class AppEnvIsTestListener extends PHPUnit_Framework_BaseTestListener

  public function startTest(PHPUnit_Framework_Test $test) {
    if (getenv("APP_ENV") !== "test") {
      echo "A prior test has changed APP_ENV!\n";

To use the Listener, you’ll need to add it to your phpunit.xml:



Combine this with the --testdox flag to phpunit and you’ll get a very basic glimpse into which of your tests is leaking environment changes, and causing a later test to fail:

$ phpunit --filter EnvTest --testdox
PHPUnit 5.7.27 by Sebastian Bergmann and contributors.

 [x] Tests a thing
 [x] Tests another thing
A prior test has changed APP_ENV!
 [x] Tests a thing that does not depend on APP_ENV
A prior test has changed APP_ENV!
 [ ] Tests a thing that depends on APP_ENV
A prior test has changed APP_ENV!
 [ ] Tests another thing that depends on APP_ENV

Above, we can now see that the Tests another thing test case is leaking a change to APP_ENV and is likely causing the last two tests to fail.