php+xml(修改xml的内容)

<?
$doc = new domDocument();
// $doc->load( 'books.xml' );
//$doc = simplexml_load_file('books.xml');
if(isset($_POST['vote']) && $_POST['vote']!=""){
$select=$_POST['select1'];
echo $select;
$renqis = $doc->documentElement;
//$renqi = $library->childNodes;
//$renqis =$book->getElementsByTagname("renqi");
//$renqi =$renqis->item(0)->nodeValue;
$xpath = new domxpath($doc);
// $firstTitle =$renqis->childNodes;
$firstTitle=$xpath->query("/rss/books/book/renqi")->item(0);
$newTitle = $doc->createElement("renqi");
$newTitle->appendChild(new DOMText('$select'));

// $newTitle = $doc->createElement("renqi");
// $newTitle->appendChild(new DOMText("66"));
$firstTitle->parentNode->replaceChild($newTitle, $firstTitle);


}
?>

怎么出现Fatal error: Call to a member function replaceChild() on a non-object in D:\www\peill\admin\xml.php on line 20错误为什么不支持replaceChild呢是不是缺少什么，请大家帮我看看谢谢