PHP操作XML有关问题

PHP操作XML问题
我用php生成xml如下

PHP code

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$xml = '<?xml version="1.0" encoding="utf-8"?>'."\r\n";
$xml .= '<user>'."\r\n";
$xml .= "\t".'<person_siteurl>'.www.163.com.'</person_siteurl>'."\r\n";
foreach (array(1,2,3) as $value){
   $xml .= "\t".'<job_keys>'.$value.'</job_keys>'."\r\n";
}
$xml .= '</user>'."\r\n";
$sxe = new SimpleXMLElement($xml);
$sxe->asXML(APPPATH.'user.xml');

生成了user.xml文件，我现在要想添加一个：
李四
该怎么添加啊，前面跟"\t" ,后面跟"\r\n" ？ 

感谢~~

------解决方案--------------------

PHP code

$sxe = new SimpleXMLElement($xml);
$sxe->addChild('username', '李四');
$sxe->asXML(APPPATH.'user.xml'); <div class="clear">
                 
              
              
        
            </div>