Identical nested XML elements with namespaces and PHP - php

Question

Try as I may, I cannot seem to grab the value of the "Id" attribute in the nested apcm:Property element, where the "Name" attribute equals "sequenceNumber", on line 12. As you can see, there element of interest is buried in a nest of other elements with an identical name and namespace.
Using PHP, I'm having a difficult time wrapping my head around how to grab that Id value.
<?xml version="1.0" encoding="utf-8" ?>
<feed xmlns="http://www.w3.org/2005/Atom" xmlns:apcm="http://ap.org/schemas/03/2005/apcm" xmlns:apnm="http://ap.org/schemas/03/2005/apnm" xmlns:georss="http://www.georss.org/georss">
<id>urn:publicid:ap.org:30085</id>
<title type="xhtml">
<apxh:div xmlns:apxh="http://www.w3.org/1999/xhtml">
<apxh:span>AP New York State News - No Weather</apxh:span>
</apxh:div>
</title>
<apcm:Property Name="FeedProperties">
<apcm:Property Name="Entitlement" Id="urn:publicid:ap.org:product:30085" Value="AP New York State News - No Weather" />
<apcm:Property Name="FeedSequencing">
<apcm:Property Name="sequenceNumber" Id="169310964" />
<apcm:Property Name="minDateTime" Value="2012-05-22T18:04:18.913Z" />
</apcm:Property>
</apcm:Property>
<updated>2012-05-22T18:04:18.913Z</updated>
<author>
<name>The Associated Press</name>
<uri>http://www.ap.org</uri>
</author>
<rights>Copyright 2012 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.</rights>
<link rel="self" href="http://syndication.ap.org/AP.Distro.Feed/GetFeed.aspx?idList=30085&idListType=products&maxItems=20" />
<entry>
...
</entry>
</feed>

Answer 1

You have to register the namespaces, and use the [] predicate to identify which Property element you are interested in. It is safest if you do NOT use double slash, i.e., if you start the look up from the document element.
<?php
$xml = <<<EOD
...
EOD;
$sxe = new SimpleXMLElement($xml);
$sxe->registerXPathNamespace('apcm', 'http://ap.org/schemas/03/2005/apcm');
$sxe->registerXPathNamespace('atom', 'http://www.w3.org/2005/Atom');
$result = $sxe->xpath('/atom:feed/acpm:Property[#Name=\'FeedProperties\']/acpm:Property[#Name=\'FeedSequencing\']/acpm:Property[#Name=\'sequenceNumber\']/#Id');
foreach ($result as $sequenceNumber) {
echo $sequenceNumber . "\n";
}
?>
Note that there may theoretically be multiple sibling Property elements with the same #Name and so this Xpath may produce multiple nodes (#Id values).