Im making a simple web-based control panel, and i figured the easiest way for me to accomplish it would be to have PHP on the 2 machines (one being the web facing machine, the other being behind a VPN), basically I need it so when I press a button on the site on the externally facing IP of machine 1, it sends a request to the internally facing IP (eg 192.168.100.1) of machine 2 and runs the PHP file (test.php plus some $_GET data) without actually redirecting the end user to 192.168.100.1, because obviously that will time out as there is no access to it.
You can use cURL to send or forward HTTP requests from machine 1 to machine 2 and to receive the responses machine 2 gives you and (if needed) process those responses to show them to the user.
You could also use (XML-/JSON-)RPC or SOAP which would be a bit more elegant and extensible (more commonplace than using cURL) but it would have a higher learning curve with a bigger setup time/work.