gistfile1.php

<?php

$p = $document->createElement('p', 'foobar');

$elements = $document->getElementsByTagName('p');
$lastElement = $elements[$elements->length - 1];

if($lastElement->nextSibling) {
    $elements->insertBefore($p, $lastElement->nextSibling);
}
else {
    $lastElement->parentNode->appendChild($p);
}