Force the session to be saved and closed.
This method must invoke session_write_close() unless this interface is used for a storage object design for unit or functional testing where a real PHP session would interfere with testing, in which case it it should actually persist the session data if required.
Overrides SessionStorageInterface::save
public function save() {
// nothing to do since we don't persist the session data
$this->closed = false;
}