<?php
$xmlString = <<<XML
<?xml version="1.0" encoding="UTF-8"?>
<GeteBayDetailsResponse xmlns="urn:ebay:apis:eBLBaseComponents">
  <Timestamp>2012-07-04T13:54:51.873Z</Timestamp>
  <Ack>Success</Ack>
  <Version>779</Version>
  <Build>E779_CORE_BUNDLED_14986049_R1</Build>
  <SiteDetails>
    <Site>US</Site>
    <SiteID>0</SiteID>
    <DetailVersion>1</DetailVersion>
    <UpdateTime>2009-07-09T10:48:17.000Z</UpdateTime>
  </SiteDetails>
  <SiteDetails>
    <Site>Canada</Site>
    <SiteID>2</SiteID>
    <DetailVersion>1</DetailVersion>
    <UpdateTime>2009-07-09T10:48:17.000Z</UpdateTime>
  </SiteDetails>
</GeteBayDetailsResponse>
XML;

$xml = simplexml_load_string($xmlString);
$xml->registerXpathNamespace('ebay','urn:ebay:apis:eBLBaseComponents');
$site = 'Canada';

$siteId = $xml->xpath('ebay:SiteDetails/ebay:Site[text()="'.$site.'"]/../ebay:SiteID');

echo (string)$siteId[0];

2