创设xml数据无法显示

创建xml数据无法显示
<?php

$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";
echo $doc->saveXML() . "\n";

echo "Saving only the title part:\n";
echo $doc->saveXML($title);

?>

echo 出来只显示
Saving all the document: Saving only the title part: 
xml的内容无法显示出来！

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