PHP DOMDocument replace DOMElement contents with HTML string

This is another StackOverflow answer I’m moving over to my blog.

AWinter asked:

Using PHP I’m attempting to take an HTML string passed from a WYSIWYG editor and replace the children of an element inside of a preloaded HTML document with the new HTML.

So far I’m loading the document identifying the element I want to change by ID but the process to convert an HTML to something that can be placed inside a DOMElement is eluding me.

$doc = new DOMDocument();
$doc->loadHTML($html);

$element = $doc->getElementById($item_id);
if(isset($element)){
    //Remove the old children from the element
    while($element->childNodes->length){
        $element->removeChild($element->firstChild);
    }

    //Need to build the new children from $html_string and append to $element
}

My answer:

If the HTML string can be parsed as XML, you can do this (after clearing the element of all child nodes):

$fragment = $doc->createDocumentFragment();
$fragment->appendXML($html_string);
$element->appendChild($fragment);

If $html_string cannot be parsed as XML, it will fail. If it does, you’ll have to use loadHTML(), which is less strict — but it will add elements around the fragment which you will have to strip.

Unlike PHP, Javascript has the innerHTML property which allows you to do this very easily. I needed something like it for a project so I extended PHP’s DOMElement to include Javascript-like innerHTML access.

With it you can access the innerHTML property and change it just as you would in Javascript:

echo $element->innerHTML;
$elem->innerHTML = 'example';