DOMDocument:saveXML 返回值替空字符串

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