First and simple answer is – session born in server and lives browser’s cookie. If cookie is not enabled, it parses to browser URL as variable (serialized).

Now, how can server remember the huge number of variables that is registered with session? Specially when PHP is installed as CGI wrapper, as PHP interpreter is created and destroyed for every page request.

Yes, server needs to save those data in its physical memory too.
Session creates a file to save its information at the location assigned by session_save_path() function or set by session.save_path option.

So, at the final sentence, we can say that session uses both site memory – server side (physical file in session_save_path location) and client side (physical file cookie) or in URL as a variable.

Thanks

Arshad

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