Add XPath methods to DOMDocument

QueryDOM.php

<?php

class QueryDOMDocument extends \DOMDocument
{
  	/** @var \DOMXPath */
	protected $xpath;

	public function __construct()
	{
		parent::__construct();
		$this->registerNodeClass('DOMNode', 'QueryDOMNode');
		$this->registerNodeClass('DOMElement', 'QueryDOMElement');
	}

	public function load($file, $options = null)
	{
		parent::load($file, $options);
		$this->xpath = new DOMXPath($this);
	}

	public function registerNamespace($prefix, $namespaceURI)
	{
		$this->xpath->registerNamespace($prefix, $namespaceURI);
	}

	public function registerPhpFunctions($restrict)
	{
		$this->xpath->registerPhpFunctions($restrict);
	}

	public function query($xpath, $contextNode = null)
	{
		return $this->xpath->query($xpath, $contextNode);
	}

	public function evaluate($xpath, $contextNode = null)
	{
		return $this->xpath->evaluate($xpath, $contextNode);
	}
}

class QueryDOMElement extends \DOMElement
{
	use DOMNodeXPath;
}

class QueryDOMNode extends \DOMNode
{
	use DOMNodeXPath;
}

trait DOMNodeXPath
{
	public function query($xpath)
	{
		return $this->ownerDocument->query($xpath, $this);
	}

	public function evaluate($xpath)
	{
		return $this->ownerDocument->evaluate($xpath, $this);
	}
}

test.php

<?php

require 'QueryDOM.php';

$doc = new QueryDOMDocument();
$doc->load('test.xml');

$doc->registerNamespace('oai', 'http://www.openarchives.org/OAI/2.0/');
$doc->registerNamespace('oai_dc', 'http://www.openarchives.org/OAI/2.0/oai_dc/');
$doc->registerNamespace('dc', 'http://purl.org/dc/elements/1.1/');

foreach ($doc->query('oai:ListRecords/oai:record/oai:metadata/oai_dc:dc') as $item) {
  	$data = array(
		'date' => $item->evaluate('string(dc:date)'),
		'source' => array(),
	);

	foreach ($item->query('dc:source') as $source) {
		$data['source'][] = $source->textContent;
	}

	print_r($data);
}

test.xml

<OAI-PMH
  xmlns="http://www.openarchives.org/OAI/2.0/"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns:dc="http://dublincore.org/documents/dcmi-namespace/"
	xsi:schemaLocation="http://www.openarchives.org/OAI/2.0/ http://www.openarchives.org/OAI/2.0/OAI-PMH.xsd">
	<responseDate>2013-04-15T12:14:31Z</responseDate>
	<ListRecords>
		<record>
			<header>
				<identifier>123</identifier>
				<datestamp>2013-08-16T14:42:52Z</datestamp>
			</header>
			<metadata>
				<oai_dc:dc
					xmlns:oai_dc="http://www.openarchives.org/OAI/2.0/oai_dc/"
					xmlns:dc="http://purl.org/dc/elements/1.1/"
					xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
					xsi:schemaLocation="http://www.openarchives.org/OAI/2.0/oai_dc/ http://www.openarchives.org/OAI/2.0/oai_dc.xsd">
					<dc:date>2013-09-16</dc:date>
					<dc:source>123</dc:source>
					<dc:source>456</dc:source>
				</oai_dc:dc>
			</metadata>
			<about></about>
		</record>
	</ListRecords>
</OAI-PMH>