I use Gitlab CI for unit testing of a PHP application that talks to a database server. We use a test matrix to test all combinations of versions of PHP and database engines that we support, and have just added MySQL 8.4 to our testing regime. It doesn’t work out of the box with PHP 7.3, but needs mysql_native_password=ON adding to my.cnf. (I appreciate it’s fairly unlikely that someone will use such an old version of PHP in conjunction with a brand new version of MySQL, but the test matrix doesn’t know that.)