Is there something extra I must do in order to get PhoneGap to handle the PHP request?
PhoneGap does not support PHP. It will not run your PHP scripts.
When your server sees a file with PHP in it, it executes the PHP, removes it from the document, and replaces it with the result of executing it. Browsers do not run PHP - your server does.
A PhoneGap app is just a browser control wrapped in a native app. Browsers don't run PHP and thus PhoneGap doesn't run PHP.
If you need to access a database on your server, send an email, etc. you can still use your server to do that. It doesn't work any differently with PhoneGap than it does with a desktop browser. You'll probably want to write a "web service" on your server to provide whatever services your app might need. You should avoid over-dependence on it, though, as requiring online access for the app to work will get a quick App Store rejection (at least for Apple.)
PhoneGap has various plugins for local database on the device - or you can use the various native Local Storage options in mobile browsers, should you need a local database.