Create custom PHP Page

Hello,

I am hoping you can help (and secretly hoping it’s a fairly easy task)

I am looking to create a custom php page which will do some processing.
This page would be accessed from the main GP/WP site using JS in Hooks.

I tried using hooks and headers for the PHP within ‘elements’ however it appears that all of the available options only process the php after some headers have started being generated, therefore meaning that the php would not present data as accurately as expected.

Example php (simplified);
<?php echo 'success'; exit(); ?>

Therefore I want the page to be as plain as possible OR to be able to execute the php early enough that the exit(); stops the actual page from being rendered into HTML (headers, etc.)

I have tried to use the “disable elements” for this page when editing however this doesn’t seem to do enough.

I am running multi-site and cannot be sure a directory on my physical server where I could place this file, in order to access it from the front-end.

Please could you help me to find a solution?

Many Thanks!