<?php

function convertHTMLtoXML($sourceHTML) {
    // XML Document
    $xmlDocument = new DOMDocument('1.0', 'utf-8'); // The our xml output document
    $xmlDocument->formatOutput = true;

// XML Document Root tag, You can replace its name with your favorite name
    $root = $xmlDocument->createElement("root");

// HTML Document
    $document = new DOMDocument('1.0', 'utf-8'); // The our html input document
    $document->loadHTML($sourceHTML);

// The Images
    $imgs = $document->getElementsByTagName("img");

// The YouTube Video
    $video = $xmlDocument->createElement("video");
    $url = $xmlDocument->createElement("url", getYoutubeURL($sourceHTML));
    $video->appendChild($url);
    $root->appendChild($video);

// Images Wrapper
    $images = $xmlDocument->createElement("images");
    foreach($imgs as $img) {

// Image Element
        $image = $xmlDocument->createElement("image");
        for ($i = 0; $i < $img->attributes->length; $i++) {
            $attribute = $img->attributes->item($i);
            $name = $attribute->name;
            $value = $attribute->textContent;
            $element = "";
            if($name == 'src') {
                $element = $xmlDocument->createElement("url", $value);
            }
            else if($name == 'alt') {
                $element = $xmlDocument->createElement("description", $value);
            }
            if (!is_string($element)) {
                $image->appendChild($element);
            }
        }
        $images->appendChild($image);
    }
    $root->appendChild($images);
    $xmlDocument->appendChild($root);

// Save it as XML
    return $xmlDocument->saveXML();
}

function getYoutubeURL($sourceHTML) {
    preg_match("/\s*[a-zA-Z\/\/:\.]*youtu(be.com\/watch\?v=|.be\/)([a-zA-Z0-9\-_]+)([a-zA-Z0-9\/\*\-\_\?\&\;\%\=\.]*)/i", $sourceHTML, $matches);
    return $matches[0];
}

$input = "https://www.youtube.com/watch?v=NVyghyOtWKw Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et
<img class=\"alignnone size-large wp-image-23904\" src=\"https://picnic.ly/URL1\" alt=\"description 1\" width=\"1024\" height=\"574\" />
<img class=\"alignnone size-large wp-image-23904\" src=\"https://picnic.ly/URL2\" alt=\"description 2\" width=\"1024\" height=\"574\" />
<img class=\"alignnone size-large wp-image-23904\" src=\"https://picnic.ly/URL3\" alt=\"description 3\" width=\"1024\" height=\"574\" />
";

echo "-------------\n";
echo "Our Input:\n";
echo "-------------\n";
echo $input;
echo "\n";
echo "-------------\n";
echo "Our Output:\n";
echo "-------------\n";
echo convertHTMLtoXML($input);
echo "\n";

Code

Result

Thats now fixed

We used domdocument for xml instead of a string.

Features:

1- Lightweight

2- Faster

3- Better than old at all.