How to get the HTTP_REFERER from a Yahoo or Gmail

I have an e-mailer script that send messages to Yahoo or Gmail with a link on it, a link that will return back to my website when it is clicked.

I need to get the HTTP_REFERER from Yahoo or Gmail. in my site I have

echo $_SERVER["HTTP_REFERER"];

expecting something to return on it. but its empty.

This seems to be by design. It is possible to remove the header using Javascript shenanigans. From Google's docs on GMail privacy:

Minimized "referrer" header information. When you click on links in messages, the web browser that loads contains a referrer header. When you click on links in Gmail, Google takes steps to eliminate this referrer header, preventing others from knowing that you clicked on a link from an email.

You have no control over HTTP_REFERER. What you see is what the client passed.

The browser might not provide HTTP_REFERER so you cannot rely on it - Secondly, some software firewalls (like Norton Internet Security) have features to strip out such information...