DOMDocument::saveXML 返回值为空字符串

        $doc = new DOMDocument('1.0');	// we want a nice output	$doc->formatOutput = true;	$root = $doc->createElement('book');	$root = $doc->appendChild($root);	$title = $doc->createElement('title');	$title = $root->appendChild($title);	$text = $doc->createTextNode('This is the title');	$text = $title->appendChild($text);			echo "Saving all the document:\n";		$xmlContent = $doc->saveXML();		var_dump($xmlContent);

Saving all the document:string(72) "<?xml version="1.0"?><book>  <title>This is the title</title></book>"