How to tell if a page is being called via Ajax, or on it's own

Question

I have a page that loads other pages via Ajax (think frames, except without the frames).

Obviously these pages can all be called independently, so I want to detect

Answer 1

Modern browsers add the following request header when a request is made using the XMLHttpRequest object:

X-Requested-With: XMLHttpRequest

In PHP, check the existence of this header using:

$_SERVER['HTTP_X_REQUESTED_WITH']