HTTP REFERER not working. Why?

Question

HTTP headers

I have a site where certain actions are done depending on $_SERVER['HTTP_REFERER'] .
And so $_SERVER['HTTP_REFERER'] is not always transferred. I understand that it is set by the browser, but I go from one browser, incognito, first to one site where there is a link to mine, I press and the referrer is transmitted, but not from the other.
In general, REFERER is not transmitted from 50% of sites, what could be the reasons?

Answer 1

here are several reasons