Let's suppose image.php is a file that outputs some image.
We include it in .html page (it is NOT server-side scripting
enabled page, just plain .html):
<img src=http://site.com/image.php>
Now, how do we RELIABLY get the url of a calling .html page ?
It looks like browser does not always send HTTP_REFERRER when
retrieving image.php, particularly when it is included on https
site page.

The problem is it is not working when I use the function on a secure server.