XML DOM & XPath

I have an XML document as a string $strXml, that looks a little like this:

<recordset>
    <row>
        <userid>1</userid>
        <username>Bob</username>
        <password>Rover</username>
    </row>
    <row>
        <userid>2</userid>
        <username>Fred</username>
        <password>Fido</username>
    </row>
</recordset>

What's the best way to grab a single value from this form? In the past (in
VB), I have loaded the XML into a DOM Document, and used an XPath query to
select the data I want, and then bung the output into a string.

Any suggestions?

Thanks
Ian