Simple, fast, easy…

SELF = if you need index.php

If you want to include something from your root folder
FCPATH = C:\xampp\htdocs\root\

If you need to include something from your application folder
APPPATH = C:\xampp\htdocs\root_folder\application\

If you need to include something from your system folder
BASEPATH = C:\xampp\htdocs\root_folder\system\

That’s it.