PHP操作XML有关问题

$xml = ''."\r\n";
$xml .= ''."\r\n";
$xml .= "\t".''.www.163.com.''."\r\n";
foreach (array(1,2,3) as $value){
   $xml .= "\t".''.$value.''."\r\n";
}
$xml .= ''."\r\n";
$sxe = new SimpleXMLElement($xml);
$sxe->asXML(APPPATH.'user.xml');

$sxe = new SimpleXMLElement($xml);
$sxe->addChild('username', '李四');
$sxe->asXML(APPPATH.'user.xml');