I need to execute PHP code as part of a webhook

I have a webhook that is expecting to hit a PHP target. I do not have full control over the web server that my Grav site is running on, but I have full control over the location that Grav was installed into. That part works. What I need is to know if I can modify the .htaccess file to allow direct PHP execution in a specific directory inside user/pages or if that will break something.

I do not have control over the name of the script that the webhook is expecting to execute, so I cannot adapt the source; I must adapt Grav.

Any help would be appreciated.