php.net:
Session support in PHP consists of a way to preserve certain data
across subsequent accesses. This enables you to build more customized
applications and increase the appeal of your web site.
A visitor accessing your web site is assigned a unique id, the
so-called session id. This is either stored in a cookie on the user
side or is propagated in the URL.
that mean is different between user because session id is different per user and Session allow user access to owner associative array
the value that is set for $_SERVER is on RAM and didn't store in file or database and it remove from RAM after request finish. for this kind of work like save a variable during of user is online you can use $_SESSION but if you want save variable for all user you can use Database