JS Script To Pass Variable and Retrieve Another JS script,

I have a program that returns JS to a site, but the JS is returned upon calling a PHP script. Google Tag Manager doesn't like the fact that the script scr is a PHP file and is throwing many errors. So I need to somehow have an externam script src file that will refer to a PHP script to process and then return the JS to the requesting site. This is for a pop up program called xzy_script. The current inserted script on my clients website looks like this: <script type="text/javascript" src="https://[URL]/[login to view URL]" ></script> I have been told that if this is inserted by GTM, it needs to be a js file, not a PHP file being used as the src. Is there a way to sent that variable $_GET['page'] to a js file, and then redirect and return the original process of the index..php file?