DOMDocument replace iframe with its src

<?php
$dom = new DOMDocument();
$dom->loadHTMLFile('https://www.youtube.com/embed/tgbNymZ7vqY');
$iframes = $dom->getElementsByTagName('iframe');
foreach($iframes as $iframe) {
    $src = $iframe->getAttribute('src');
    if (strpos($src, 'embed') !== false) {
        $iframe->parentNode->replaceChild(new DOMText($src), $iframe);
    }
}
echo $dom->saveHTML();