how to format xml file by php dom

When I try to open xml file by editor like notepad. The format of xml file like this...

<numbers><number>1</number><number>2</number><number>3</number></numbers>

I want it can be...

<numbers>
    <number>1</number>
    <number>2</number>
    <number>3</number>
<numbers>

in .php file

...
$dom = new DOMDocument();
$dom->preserveWhiteSpace = true;
$dom->formatOutput = true;
$dom->loadXML($xml->asXML());
$dom->save('number.xml');