public function read($sid)
    {
        //save the session ID for closing later
        $this->sid = $sid;

        //a lock MUST be acquired, but the complexity is in the lock() method
        $this->lock($sid);

        $this->sessionDoc = $this->sessions->findOne(array('_id' => $sid));

        if (!$this->sessionDoc) {
            return '';
        } else {
            //return the string data (stored as Mongo binary format)
            return $this->sessionDoc['data']->bin;
        }
    }