<?php
$dom = new DOMDocument();
$dom->loadXML('<?xml version="1.0"?><container><p>foo</p></container>');
$container = $dom->documentElement;
$p = $container->firstElementChild;
$p->insertAdjacentElement('beforebegin', $dom->createElement('A'));
echo $dom->saveXML();
?>
<?xml version="1.0"?>
<container><A/><p>foo</p></container>