DOMDocument:saveXML 返回值替空字符串

DOMDocument::saveXML 返回值为空字符串
PHP手册中的的实例：

        $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) "

  

"