PHPUnit aggressively drops supported PHP versions, which makes it difficult to test on PHP 7.0 all the way to PHP 8, as newer PHPUnit versions are required to test on newer PHP versions, but the tests cannot be kept compatible between PHPUnit 6 (required for PHP 7.0) and PHPUnit 9 (required for PHP 8.0).

The Symfony PHPUnit Bridge solves this problem by not making PHPUnit a direct dependency, but rather installing the correct version on demand. On top of that, it offers and injects several traits that allow us to run the same test code on all PHP versions we currently support.