In the meantime, here's a utility function that you may find helpful:

$innerHTML = '';
$elem = $doc->getElementById($elem_id);

// loop through all childNodes, getting html		
$children = $elem->childNodes;
foreach ($children as $child) {
    $tmp_doc = new DOMDocument();
    $tmp_doc->appendChild($tmp_doc->importNode($child,true));		
    $innerHTML .= $tmp_doc->saveHTML();
}

I was trying to use the above method to get the innerHTML in an HTML document but sine these functions are designed for XML, some HTML will get changed. Example, <br /> will get changed to <br>

I really hope the PHP Developers will see the need for this feature and add it in.

You can try using this the DOM features found over at http://raxanpdi.com to see if they can assist you.

The innerHTML property is not part of the spec and will not be added.

There is a simpler code snippet than the one in the comments before using saveXML with a node argument:

$innerHtml = implode("\n", array_map(function ($node) {
    return $node->ownerDocument->saveXML($node);
}, iterator_to_array($d->documentElement->childNodes));

With https://github.com/php/php-src/pull/4709 which will most likely go into PHP 8 there is a simple three liner liner that allows this:

    $fragment = $dom->createDocumentFragment(); 
    $fragment->append(...$element->childNodes);
    echo $dom->saveXML($fragment);

This will modify the XML doc though and remove the child nodes from $element.