public function start($type = '*firefox', $startUrl = 'http://localhost')
    {
        if (null !== $this->sessionId) {
            throw new Exception("Session already started");
        }

        $response = $this->doExecute('getNewBrowserSession', $type, $startUrl);

        if (preg_match('/^OK,(.*)$/', $response, $vars)) {
            $this->sessionId = $vars[1];
        } else {
            throw new Exception("Invalid response from server : $response");
        }
    }