illegal self closing node notation for empty nodes - outputting XHTML with PHP DOMDocument

I am processing an XML compliant input of XHTML using XPATH in PHP like this:

$xml=new DOMDocument();
$xml->loadXML(utf8_encode($temp));
[...]
$temp=utf8_decode($xml->saveXML());

The problem that arises is that nodes that may not be self closing according to the HTML5 specs, e.g.

<textarea id="something"></textarea>

or a div to leverage by JS

<div id="someDiv" class="whaever"></div>

come back out as

<textarea id="something" />

and

<div id="someDiv" class="whaever" />

I currently address this by using str_replace, but that's nonsese as I need to match individual cases. How can I solve this?

At the same time XPATH insists on putting out

xmlns:default="http://www.w3.org/1999/xhtml

and on individual nodes freshly created, it puts stuff like <default:p>. How do I stop that without resorting to stupid search and replace like this:

$temp=str_replace(' xmlns:default="http://www.w3.org/1999/xhtml" '," ",$temp);
$temp=str_replace(' xmlns:default="http://www.w3.org/1999/xhtml"'," ",$temp);
$temp=str_replace('<default:',"<",$temp);
$temp=str_replace('</default:',"</",$temp);

EDIT: I'm really getting trouble with the stupid search and replace and I do not intend to attack the output XHTML with RegExp. Consider this example:

<div id="videoPlayer0" class="videoPlayerPlacement" data-xml="video/cp_IV_a_1.xml"/>

Obviously self-closing divs are illegal (at least in one context where I cannot output as mime application/xhtml+xml but am forced to use mime text/html) and in all other cases they sure don't validate.

Solution

It is possible to normalize "non void" tags using a trick. It is not an official solution, but it works.

function export_html(DOMDocument $dom)
{
    $voids = [
        'area',
        'base',
        'br',
        'col',
        'colgroup',
        'command',
        'embed',
        'hr',
        'img',
        'input',
        'keygen',
        'link',
        'meta',
        'param',
        'source',
        'track',
        'wbr',
    ];

    // Every empty node; 
    // there is no reason to match nodes with content inside.
    $query = '//*[not(node())]';
    $nodes = (new DOMXPath($dom))->query($query);

    foreach ($nodes as $node) {
        if (in_array($node->nodeName, $voids)) {
            // A void tag.
            continue;
        }
        // Not a void tag. We inject a placeholder content.
        $node->appendChild(new DOMComment('NOT_VOID'));
    }
    
    // We remove the placeholders.
    return str_replace('<!--NOT_VOID-->', '', $dom->saveXML());
}

In your example

$dom = new DOMDocument();
$dom->loadXML(<<<XML
<html>
    <textarea id="something"></textarea>
    <div id="someDiv" class="whaever"></div>
</html>
XML
);

echo export_html($dom); will produce

<?xml version="1.0"?>
<html>
    <textarea id="something"></textarea>
    <div id="someDiv" class="whaever"></div>
</html>