What is the most reliable and secure way to determine what page either sent, or called (via AJAX), the current page. I don't want to use the $_SERVER['HTTP_REFERER'], because of the (lack of) reliability, and I need the page being called to only come from requests originating on my site.
Edit: I am looking to verify that a script that preforms a series of actions is being called from a page on my website.
The REFERER is sent by the client's browser as part of the HTTP protocol, and is therefore unreliable indeed. It might not be there, it might be forged, you just can't trust it if it's for security reasons.
If you want to verify if a request is coming from your site, well you can't, but you can verify the user has been to your site and/or is authenticated. Cookies are sent in AJAX requests so you can rely on that.