How do I add a link into an xml tag using php

I have an xml file that I can pull using php and display the results. I need to pick up one of the xml tags and add a link inside it so I can parse the value in the tag as a php variable. e.g.

<name>Llandaff</name>
<firmid>70330</firmid>
<branchid>2</branchid>
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I want to add a link so the new xml file would be:

<name>Llandaff</name>
<firmid><a href="index.php?firmID=70330"></a></firmid>
<branchid>2</branchid>
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