&$this in WordPress add_action callback

Solution:

Yes – you are safe to just use array($this, 'init');.

What this is actually is, is a “callable” in PHP. It will be invoked using call_user_func() or call_user_func_array() (internally inside the add_action method).

PHP’s official description of this type of callable:

A method of an instantiated object is passed as an array containing an object at index 0 and the method name at index 1.

You can read more about callables here.