PHP和xPath问题

<?php
$xml = <<<XML<books>
  <book>
   <title>Book 1</title>
   <author>Author 1</author>
  </book>
  <book>
   <title>Book 2</title>
   <author>Author 2</author>
  </book>
</books>
XML;

$dom = new DOMDocument();
$dom->loadXML($xml);

$xpath = new DOMXPath($dom);

$books = $xpath->query("//book");

foreach ($books as $book) {
  $title = $xpath->query("title", $book)->item(0)->nodeValue;
  $author = $xpath->query("author", $book)->item(0)->nodeValue;

  echo "Title: " . $title . ", Author: " . $author . "\n";
}
?>