How to show an error message if a (html) file is not parsed by PHP?

So I have a html file including some php code. As I offer it for downloading (because it is in an open source project) you may imagine that some people try to access the file directly, e.g. by drag and dropping it into their browser.

Obviously this is not correct and may cause errors, because the PHP code cannot be executed. So most browsers just ignore the PHP code and display the HTML file like it is displayed without any error message.
So basically my aim is to show my own error message there only if the file is not parsed by PHP.