include()for when if the file isn’t found or loaded the page will still function or load without it.
include_once()if you don’t want errors to make this file be loaded twice and slow down requests. Or if it has functions that may be re declared twice.
require() is for when you must have this file, stop loading if the file is not present or has errors. Use this for important variables like DB connectors.
require_once()is used for one time loading like functions that cannot be re declared. Do note that this is slower than just