<?php
 
$xml = <<<'EOT'
<?xml version="1.0"?>
<stuff>
    <other_nodes>
    </other_nodes>
    <images>
        <image><sorted_number><![CDATA[1]]></sorted_number></image>
        <image><sorted_number><![CDATA[3]]></sorted_number></image>
        <image><sorted_number><![CDATA[2]]></sorted_number></image>
    </images>
</stuff>
EOT;
 
$dom = new DOMDocument();
$dom->loadXML( $xml, LIBXML_NOBLANKS );
$dom->formatOutput = True;
 
$images = $dom->getElementsByTagName( 'image' );
 
/* This is the same as your array conversion: */
$sorted = iterator_to_array( $images );
 
/* This is your sorting routine adapted to DOMDocument: */
usort( $sorted, function( $a, $b )
{
    return
    $a->getElementsByTagName('sorted_number')->item(0)->nodeValue
    -
    $b->getElementsByTagName('sorted_number')->item(0)->nodeValue;
});
 
/* This is the core loop to “replace” old nodes: */
foreach( $sorted as $node ) $images->item(0)->parentNode->appendChild( $node );
 
echo $dom->saveXML();
 

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