If you want to create nested DOM elements:
<?php
$doc = new DOMDocument();
$foo = $doc->createElement("foo");
$doc->appendChild($foo);
$bar = $doc->createElement("bar");
$foo->appendChild($bar);
$bazz = $doc->createElement("bazz");
$foo->appendChild($bazz);
echo $doc->saveXML();
?>
Is equivalent to:
<foo>
<bar></bar>
<bazz></bazz>
</foo>