Show header only to specific groups|Show/Hide PHP code dependent on what group you are in

I’m trying to hide the NC header to users belonging to a group, I created a theme and added the PHP file to that, my thinking was to add a PHP if statement to show/hide the header element.
I was thinking of using the function isInGroup($userId, $group). how can I get the current userid?
I’m very new to NextCloud and know practically nothing about PHP. If anyone knows if this is the correct use of the function or how to do this your help would be greatly appreciated.