I think there is some kind of conceptual error (at least when session.auto_start=0). I was trying to use Stack\Session and always got an empty session ID.

Reason is, that $session->migrate is called when no cookie is present. This leads to a call to session_regenerate_id (which fails, because no session was started yet). Afterwards NativeSessionStorage::loadSession is invoked, which sets NativeSessionStorage::$started to true.

So in the end I have a broken session (empty session id). I could fix this, by adding a $session->start() before the call to $session->migrate().