A WordPress plugin I’ve been working on recently needed to be able to accept configuration in a few different ways: users should be able to define a constant in the wp-config.php file or fill out a form within a settings screen. If the constant is defined, the setting screen should be aware of that and hide the setting, since the constant should take precedence.

This is a pretty common pattern in WordPress plugins, but it can get rather tricky to test; by design, once a constant is defined in PHP, you shouldn’t be able to change its value. PHPUnit has ways to work around this by running tests that define constants in separate processes, but this can seriously impact the performance of your test suite. Furthermore, the WordPress core test suite is pretty tightly coupled, so it doesn’t like when tests are run separately.

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