Automatic (still using old PHPUnit versions). Verify that CIs (ToBiC comment in this issue) continue passing. Verify that the GHA badges in this issue (near the branches information @ the top) continue passing. Manual (forcing newer PHPUnit versions to be used). Note 1: Following numbered steps need to be repeated for the 4 branches (39, 311, 400 and master). Note 2: Use PHP 7.4 for all them (to avoid having to switch of PHP versions). Testing steps Delete the composer.lock file and the vendor directory completely: rm -fr composer.lock vendor Init PHPUnit: php admin/tool/phpunit/cli/init.php Verify that you see the following in the output: For 39_STABLE: phpunit/phpunit (7.5.20) as part of the process. It's important the "20" (that is the minimum to see the problem, so 21, 22... are ok too). sebastian/comparator (3.0.5) (or later). For the other branches: phpunit/phpunit (9.5.24) as part of the process. t's important the "24" (that is the minimum to see the problem, so 25, 26... are ok too). sebastian/comparator (4.0.8) (or later). Run this test: vendor/bin/phpunit question/type/multianswer/tests/question_test.php Verify it ends without errors. Run this test: For 39_STABLE: vendor/bin/phpunit question/behaviour/deferredcbm/tests/behaviourtype_test.php Fo the other branches: vendor/bin/phpunit question/behaviour/deferredcbm/tests/behaviour_type_test.php Verify it ends without errors.