Accessing Main file from custom class

Okay, I know these questions are STUPID, but I’m just learning.

If I have a public function in a ‘main’ file, can I trigger a function their class file.

public function main(){
public function doStuff(){

//in another file
public function customClass(){
                      //main just triggered me!
                      //how do I trigger doStuff, just using 'doStuff()' doesn't work...argh!

Also, is the term ‘main file’ the proper term for the uh…main file. I feel stupider saying it.