<?php

$xml = <<<XML
<root>
<region id='thisRegion'></region>
<region id='thatRegion'></region>
</root>
XML;

$partials['thisRegion'] = "<p>Here's this region</p>";
$partials['thatRegion'] = "<p>Here's that region</p>";
$DOM = new DOMDocument;
$DOM->loadXML($xml);
$regions = $DOM->getElementsByTagname('region');

$regionsCount = $DOM->getElementsByTagName('region')->length;
for($i= $regionsCount;$i>0;--$i)
{   
    $region=$DOM->getElementsByTagName('region')->item($i-1);
    $id = $region->getAttribute('id');
    $partial = $DOM->createDocumentFragment();
    $partial->appendXML( $partials[$id] );
    $region->parentNode->replaceChild($partial, $region);
}

echo $DOM->saveXML();
?>

<?xml version="1.0"?>
<root>
<p>Here's this region</p>
<p>Here's that region</p>
</root>