Referencing a class from another class [PHP]?

Hi! I’m trying to find out the best way to reference a class from within another class, IE: I have a base class called base, and I have a users class called users,

Within users theres: _uidToUname() which uses $this->_userExists();

I want to be able to use _uidToUname from Base.

Now If I do:
Users::_uidToUname(1); I get a, “Can’t use $this” error.

I don’t necessarily want to extend base to users.

Is there another way, or which is the best method to use?

Thanks!