public function myAction(Request $request) {
 $id = $request->getSession()->getId();
}

public function myAction(Request $request) {
 $session = $request->getSession();
 $session->start();
 $id = $session->getId();
}

public function myAction(Request $request) {
 $session = new SymfonyComponentHttpFoundationSessionSession;
 $session->start();
 $id = $session->getId();
}