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);

打印出的结果是：
Saving all the document: string(72) " "

为什么没有打印出完整的XML？请各位前辈指点，Thanks! PHP saveXML
[解决办法]
请在文本方式下查看

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