You need this declaration in the test classes. The issue is that our front controllers (www/index.php and cli/tracker.php) leave variables in global scope (the DI container and path to the autoload.php file in both; the Composer autoloader instance in CLI; and the web application instance for the web requests), so when PHPUnit tries to save state and serialize the globals those variables are included in it and the container can't be serialized when it has Closures within it.

One thing that could help with that is using an IIFE in the front controllers which would cause those variables to not be globally scoped anymore.

Not sure if a simple "Thanks" will be enough, but if I buy you a beer instead you might become an alcoholic pretty soon ?

Actually I had the declaration right here but since I also set $runTestInSeparateProcess = true; it didn't work (only changed the error message...)

The "IIFE" thing looks scary to me at first glance (in PHP) - have to get my head around it...

well, Thank you Michael!

The "IIFE" thing is pretty similar to JavaScript, https://github.com/joomla/jissues/blob/master/www/media/js/jtracker.js is exactly that ?

At a glance this all looks fine.

@zero-24 could you take a look at this?
Especially the definitions here
and also the unit test here
and see if it's understandable and maintainable?

Looks good. Maybe we can add some more Information to the dock block?

Makes sense - I added some words.

Any suggestions are welcome ?

So I sync'd this up with master and I also added matches for the PostgreSQL and SQL Server database classes to those categories (with test cases). At a glance things look good to go here for me.