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);
'; 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.