XML : How can i change tagname which has childs?

In php I have a xml file. I ask for choosing a node by using domdocument object in my file then changing its name.For example I want to change invite tag.But I can't.How can I solve my problem?

  <library>    <data a:foo="1" x="bar" xmlns:a="http://example.com/a">      <invite>        <username>jmansa</username>        <userid>1</userid>      </invite>      <update>1</update>    </data>  </library>