Prepending data to XML file
$xmldoc = new DOMDocument();
$xmldoc->load('sample.xml');



$thumbnail = $xmldoc->createElement('thumbnail');
$thumbnail->setAttribute('preview', 'This is a preview');
$thumbnail->setAttribute('previewURL', 'This is a URL');
$thumbnail->setAttribute('thumb', 'This is a Thumb');

$title = $xmldoc->createElement('title');
$title->appendChild($xmldoc->createCDATASection('This is Title'));
$thumbnail->appendChild($title);


$description = $xmldoc->createElement('description');
$description->appendChild($xmldoc->createCDATASection('This is Description'));
$thumbnail->appendChild($description);


$xmldoc->getElementsByTagName('thumbnails')->item(0)->appendChild($thumbnail);
$xmldoc->save('sample.xml');

<?php

$xmldoc = new DOMDocument();
$xmldoc->load('sample.xml');



$thumbnail = $xmldoc->createElement('thumbnail');
$thumbnail->setAttribute('preview', 'This is a preview');
$thumbnail->setAttribute('previewURL', 'This is a URL');
$thumbnail->setAttribute('thumb', 'This is a Thumb');

$title = $xmldoc->createElement('title');
$title->appendChild($xmldoc->createCDATASection('This is Title'));
$thumbnail->appendChild($title);


$description = $xmldoc->createElement('description');
$description->appendChild($xmldoc->createCDATASection('This is Description'));
$thumbnail->appendChild($description);
$thumbs = $xmldoc->getElementsByTagName('thumbnails')->item(0);
$first_thumb = $thumbs->getElementsByTagName('thumbnail')->item(0);

$thumbs->insertbefore($thumbnail, $first_thumb);

$xmldoc->save('sample.xml');
?>