Handling multiple Php Scripts that reference each other



I have a couple of php scripts that are in a single folder. One of these scripts has constants that are used in all the scripts . They reference each other using “require_once”. Which(directory) should I put these scripts and how can I call them? Or is there another method to handle scripts that are in a folder?
I know one can make scripts using Services, do I have to make a service for each script? Will they be able to reference each other?