In other words, including a .php file across an HTTP stream is the same (in principle) as opening up your favorite browser, typing in that URL, and viewing the source of the page. Of course you can't see the actual PHP source code - that would be a security vulnerability!
Also, when posting PHP code, please use the board's [PHP][/PHP] bbcode tags as they make your code much easier to read and analyze; you should go back and edit your post to add these tags.