In Plesk mysqli was activated but not working.

To check what modules are installed, you either can use:

echo phpinfo(INFO_GENERAL | INFO_GENERAL | INFO_MODULES | INFO_ENVIRONMENT);

or

echo ‘

';
    print_r(get_loaded_extensions());
    echo '

‘;

There was no mysqli, pdo_mysql or any other mysql releated .so file loaded.

To find out what prevented it from loading, i navigated to the correct php dir and ran:
php -i | grep mysqli
This showed me an error that something was missing to run mysqli.

It turned out that mysqlnd also needed to be active for mysqli and pdo_mysql to run.

So i activated mysqlnd and everything worked fine.