protected function getBaseUrl()
    {
        if (empty(self::$URL)) {
            throw new Exception(
                'You must first set the Piwik Tracker URL by calling
                 PiwikTracker::$URL = \'http://your-website.org/piwik/\';'
            );
        }
        if (strpos(self::$URL, '/piwik.php') === false
            && strpos(self::$URL, '/proxy-piwik.php') === false
            && strpos(self::$URL, '/matomo.php') === false
            && strpos(self::$URL, '/proxy-matomo.php') === false
        ) {
            self::$URL .= '/piwik.php';
        }

        return self::$URL;
    }