How to use function from mybb in custom file?

[Qiao]

I need to custom move threads.

I see "move_threads()" functions in class_moderation.php that I want to use.
How do I include it in empty php file?

I tried
include('inc/init.php');
include('global.php');
but it returns "Please make sure IN_MYBB is defined".

How it should be done?