public function xml(array $data, $charset = null)
{
$dom = new DOMDocument('1.0', $charset === null ? Yii::$app->charset : $charset);
$root = new DOMElement('xml');
$dom->appendChild($root);
$this->buildXml($root, $data);
$xml = $dom->saveXML();
return trim(substr($xml, strpos($xml, '?>') + 2));
}