add xml node at the first of the it's parent node
    <?xml version="1.0" encoding="utf-8"?>
<users>
  <user>
    <firstname>Mark</firstname>
    <surname>Zhu</surname>
    </user>
</users>

this is the user node I want to add in the front of the existing one

      <user>
    <firstname>Andy</firstname>
    <surname>Li</surname>
    </user>

The SimpleXMLElement::addChild can add the in the back of the existing one, 
Is there anybody know how to add in the front?
add xml node at the first of the it's parent node
===

    <?xml version="1.0" encoding="utf-8"?>
<users>
  <user>
    <firstname>Mark</firstname>
    <surname>Zhu</surname>
    </user>
</users>

this is the user node I want to add in the front of the existing one

      <user>
    <firstname>Andy</firstname>
    <surname>Li</surname>
    </user>

The SimpleXMLElement::addChild can add the in the back of the existing one, 
Is there anybody know how to add in the front?