What is the 100% way to determine that the user came from popular PS / directly?

Question

JavaScript

What is the 100% way to determine that the user came from popular PS / directly?

Document.referrer and $_SERVER['HTTP_REFERER'] don't always return PS, do they? Or at the current moment in relation to PS, these two values ​​always correspond to reality?

PS the question concerns all popular browsers.

Answer 1

HTTP_REFERER is given by the client (browser) as a request header.
And yes, it may not exist and the headers can always be changed, so there is no 100% way