如何使用PHP循环访问Dom并查找data-id

I have the following issue. I want to read in an external HTML file. I only need the HTML Elements between the body Tag. For that I have created below PHP Code and it works. I now need to add one more functionality, when I loop through I need to check if the HTML Element got a data-id and if so, then I have to add a value to that node. The Value is saved in a PHP Array and the Key is the same as the data-id. Any hint how I could do that? I would think that it needs to go into the foreach ($children as $child) loop, is that right? And if so, how can I check for the data-id? Any hint would be highly appreciated.

 $doc = new DOMDocument();
    $doc->loadHTMLFile("extracontent.html");

function DOMinnerHTML(DOMNode $element,$attr) 
{ 
  $innerHTML = ""; 
  $children  = $element->childNodes;

foreach ($children as $child) 
{
    $innerHTML .= $element->ownerDocument->saveHTML($child);
}

    return $innerHTML; 
}  


$oldDom = $doc->getElementsByTagName("body"); 

foreach ($oldDom as $element) 
{ 
   $newDom = DOMinnerHTML($element); 
} 

I'd go with first looking for all child nodes within body which have data-id attribute using xpath queries then appending to them or building an array:

$dom = new DOMDocument();
$dom->loadHTML($html, LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD);
$dox = new DOMXPath($dom);
$nodes = $dox->query('/html/body//*[@data-id]');
$array = [];
foreach ($nodes as $key => $node) {
    $node->appendChild(new DOMText('add text'));
    $array[$key] = ['data-id' => 'add text'];
}
echo $dom->saveHTML();