So how do this work? PHP allows you to define something and you can then check to see if it is defined somewhere else in the code.

Should be one of the very first, if not the first, lines in your main file.

// Let's define it!
define('ABC');

Place this at the top of every other page that you want to be forced through the main one.

if(!defined('ABC'))
	die();

Although it seems so simple, you’d be surprised at how many scripts aren’t protected. WordPress, for instance, doesn’t which allows you to look at theme files amongst other things.