/**
  * Initialize the root XML node [optional]
  * @param $version
  * @param $encoding
  * @param $format_output
  */
 public static function init($version = '1.0', $encoding = 'UTF-8', $format_output = true)
 {
     self::$xml = new DomDocument($version, $encoding);
     self::$xml->formatOutput = $format_output;
     self::$encoding = $encoding;
 }