public function getXML( $version = '1.0', $encoding = 'UTF-8', $willFormatOutput = true )
    {
        if (is_null( self::$instance )) {
            self::$instance               = new \DOMDocument( $version, $encoding );
            self::$instance->formatOutput = $willFormatOutput;
        }

        return self::$instance;
    }