Securing PHP using a gateway that consults a whitelist
Remove the additional information you are providing in your die statements. Just simply die quietly without revealing any information. It's similar to putting a zero byte index.html file in directories that you don't want people browsing.

Edit: Going away quietly without providing any information gives less information about your site to someone who may be trying to attack it. Any clues that you give them, like file not found for example, lets them know that they need to change their attack vector. The less information you can give them the better.