Description:
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current limitation:
session_id() needs to be called before session_start() for that purpose(change sid)

but session_regenerate_id() can be called AFTER session_start();

so why not make session_id($sid) the same way?
session_id($sid) use old scheme before session_start(); (BC)
and do update SID/send cookie after session_start() (new feature)

the reason to do this is: i'd have same userid use same session, the new one will search for old sessionid (which is better done in session read callback), and session_id($oldsid)