Debugging a POST request that comes from a remote server

Here is the scenario.  I am looking to debug a script that I wrote that handles POST requests from a remote server which I have zero access.  Basically, we have a service that sends text messages to customers, when they respond our provider sends our web site a POST request.  The script I wrote takes the data from this POST request and logs the information that is sent.  I want to debug my script but I am unsure of how or which debugging configuration I should use.  I've tried several of the different PHP configurations but I can't get it it work.

My expected behavior is the following:

When I respond to the text message the provider service sends a POST request to the script.

I set a breakpoint at the beginning of the script and when the script receives the POST request, I can step through the script.

This is what happens:

The script runs and I am unable to step through the script.

Which PHP test should I be using?  None seems to work and I am at my wits end.  Thanks for your help!!