/**
* Convert an Array to XML
* @param string $node_name - name of the root node to be converted
* @param array $arr - aray to be converterd
* @return DomDocument
*/
public static function &createXML($node_name, $arr = array())
{
$xml = self::getXMLRoot();
$xml->appendChild(self::convert($node_name, $arr));
self::$xml = null;
// clear the xml node in the class for 2nd time use.
return $xml;
}