Why is HTTP_REFERER empty?

Question

HTTP headers

Hello. In the project I write down referrers. That is, I collect statistics of sources from which users come to us. I noticed that very often HTTP_REFERER is empty. For example, I insert my link in Trello or Google Docs, immediately follow this link and the referrer is empty.
How to be? How to properly collect referral sources and why are they empty?

Answer 1

https://www.e-nor.com/blog/google-analytics/https-...
By default, such processing.
You can also optionally prohibit allowing the referrer to be passed or doing a bunch of redirects so that the referrer is lost.
In general, it is far from always present and it depends on the site from which the transition was to a greater extent.

Answer 2

Read here php.net/manual/en/reserved.variables.server.php