Napríklad takto:

$html = '<h1>nadpis</h1><div><div><p>text</p><p>text2</p></div></div>';
$doc = new DOMDocument();
$doc->loadXML("<root>$html</root>");
$doc->preserveWhiteSpace = FALSE;
$doc->formatOutput = TRUE;
foreach ($doc->documentElement->childNodes as $node)
        echo $doc->saveXML($node) . "\n";