Prerequisites This needs the tester ability to switch between php-mongo extension versions. MongoDB is installed & running. This needs to be tested with PHP 7.4, 8.0 and 8.1 Testing MOODLE_401_STABLE Using PHP 7.4, init phpunit ( php admin/tool/phpunit/cli/init.php ). Verify that you are using the php-mongodb extension 1.14.x version ( php --re mongodb | head -1 ). If needed, proceed to install it from PECL if not installed locally. I'm afraid but I don't know how to do this with Linux, with my Mac I was able to do it without too much problem with my package manager. Run vendor/bin/phpunit -v cache/stores/mongodb/tests/store_test.php Verify that there are 2 tests passed and there isn't any PHP notice/warning related with mongodb in the output or in the system logs. Using the same method followed in step #3, proceed to install from PECL the php-mongodb extension 1.15.x version Verify that you're using now the php-mongodb extension 1.15.x version ( php --re mongodb | head -1 ). Run vendor/bin/phpunit -v cache/stores/mongodb/tests/store_test.php Verify that there are 2 tests passed and there isn't any PHP notice/warning related with mongodb in the output or in the system logs. Repeat the points 2-9 above but switching to PHP 8.0 and PHP 8.1 Testing master Using PHP 7.4, init phpunit ( php admin/tool/phpunit/cli/init.php ). Verify that you are using the php-mongodb extension 1.14.x version ( php --re mongodb | head -1 ). If needed, proceed to install it from PECL if not installed locally. I'm afraid but I don't know how to do this with Linux, with my Mac I was able to do it without too much problem with my package manager. Run vendor/bin/phpunit -v cache/stores/mongodb/tests/store_test.php Verify that there are 2 tests skipped (that's because master requires extension version 1.15.0 and we are using 1.14.x). Using the same method followed in step #3, proceed to install from PECL the php-mongodb extension 1.15.x version Verify that you're using now the php-mongodb extension 1.15.x version ( php --re mongodb | head -1 ). Run vendor/bin/phpunit -v cache/stores/mongodb/tests/store_test.php Verify that there are 2 tests passed and there isn't any PHP notice/warning related with mongodb in the output or in the system logs. Repeat the points 2-9 above but switching to PHP 8.0 and PHP 8.1   Troubleshooting If the instance error when running the PHPUnit testing, make sure you already add this line in the config.php define( 'TEST_CACHESTORE_MONGODB_TESTSERVER' , 'mongodb://some-mongo:27017' ); Change this 'mongodb://some-mongo:27017' with your configuration.