GamiDPC

<?php
$content = '<div class="keep-me">Keep this div</div><div class="remove-me" id="test">Remove this div</div>';

$dom = new DOMDocument;
libxml_use_internal_errors(true);
$dom->loadHTML($content);
libxml_clear_errors();
$xPath = new DOMXpath($dom);

$domNodeList = $xPath->query('//div[@class="remove-me"]/@id');

$ids = []; // container of deleted elements
foreach ( $domNodeList as $domElement ) {
    $ids[] = $domElement->nodeValue;
}
echo $ids[0];
?>

Uncaught TypeError: Cannot read property 'innerHTML' of null
    at <anonymous>:5:92
    at Array.forEach (<anonymous>)
    at <anonymous>:4:12