Getting exsl:node-set to work in PHP

I have the following PHP code, but it's not working. I don't see any errors, but maybe I'm just blind. I'm running this on PHP 5.3.1.

<?php
$xsl_string = <<<HEREDOC
<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet version="1.0"
                xmlns="http://www.w3.org/1999/xhtml"
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                xmlns:exsl="http://exslt.org/common"
                extension-element-prefixes="exsl">
  <xsl:template match="/">
    <p>Hello world</p>
    <xsl:variable name="person">
      <firstname>Foo</firstname>
      <lastname>Bar</lastname>
      <email>test@example.com</email>
    </xsl:variable>
    <xsl:value-of select="exsl:node-set(\$person)/email"/>
  </xsl:template>
</xsl:stylesheet>
HEREDOC;

$xml_dom = new DOMDocument("1.0", "utf-8");
$xml_dom->appendChild($xml_dom->createElement("dummy"));

$xsl_dom = new DOMDocument();
$xsl_dom->loadXML($xsl_string);

$xsl_processor = new XSLTProcessor();
$xsl_processor->importStyleSheet($xsl_dom);
echo $xsl_processor->transformToXML($xml_dom);
?>

This code should output "Hello world" followed by "test@example.com", but the e-mail portion doesn't appear. Any idea what's wrong?

-Geoffrey Lee