XML Xpath Failing on getElementsByTagName - php

Question

<?xml version="1.0" encoding="UTF-8"?>
<AddProduct>
<auth><id>vendor123</id><auth_code>abc123</auth_code></auth>
</AddProduct>
What am I doing wrong to get : Fatal error: Call to undefined method DOMNodeList::getElementsByTagName()
$xml = $_GET['xmlRequest'];
$dom = new DOMDocument();
#$dom->loadXML($xml);
$xpath = new DOMXPath($dom);
$auth = $xpath->query('*/auth');
$id = $auth->getElementsByTagName('id')->item(0)->nodeValue;
$code = $auth->getElementsByTagName('auth_code')->item(0)->nodeValue;

Answer 1

You could retrieve the data (in the XML you posted) you want using XPath only:
$id = $xpath->query('//auth/id')->item(0)->nodeValue;
$code = $xpath->query('//auth/auth_code')->item(0)->nodeValue;
You are also calling getElementsByTagName() on $auth (DOMXPath), as #Ohgodwhy pointed out in the comments, which is causing the error. If you want to use it, you should call it on $dom.
Your XPath expression returns the auth child of the current (context) node. Unless your XML file is different, it's clearer to use one of:
/*/auth # returns auth nodes two levels below root
/AddProduct/auth # returns auth nodes in below /AddProduct
//auth # returns all auth nodes

Answer 2

This is what I came up with after reviewing php's documentation (http://us1.php.net/manual/en/class.domdocument.php, http://us1.php.net/manual/en/domdocument.loadxml.php, http://us3.php.net/manual/en/domxpath.query.php, http://us3.php.net/domxpath)
$dom = new DOMDocument();
$dom->loadXML($xml);
$id = $dom->getElementsByTagName("id")->item(0)->nodeValue;
$code = $dom->getElementsByTagName("auth_code")->item(0)->nodeValue;
As helderdarocha and Ohgodwhy pointed out, the getElementByTagName is a DOMDocument method not a DOMXPath method. I like helderdarocha's solution that only uses XPath, the solution I posted accomplishes the same thing but only uses the DOMDocument.