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