Why not just:
simplexml_import_dom(dom_import_simplexml($SimpleXmlNode))(PHP 5, PHP 7, PHP 8)
simplexml_import_dom — Get a SimpleXMLElement object from an XML or HTML node
$node, ?string $class_name = SimpleXMLElement::class): ?SimpleXMLElementThis function takes a node of a DOM document and makes it into a SimpleXML node. This new object can then be used as a native SimpleXML element.
nodeA DOM Element node
class_nameYou may use this optional parameter so that simplexml_import_dom() will return an object of the specified class. That class should extend the SimpleXMLElement class.
   Returns a SimpleXMLElement or null on failure.
  
   Throws a TypeError when passed a
   non-XML or non-HTML node.
  
| Версія | Опис | 
|---|---|
| 8.4.0 | Added support for Dom\Document. | 
| 8.4.0 | This function now throws a TypeError instead of
       a ValueError when passed a
       non-XML or non-HTML node. | 
Приклад #1 Importing a DOMDocument
<?php
$dom = new DOMDocument;
$dom->loadXML('<books><book><title>blah</title></book></books>');
if (!$dom) {
    echo 'Error while parsing the document';
    exit;
}
$s = simplexml_import_dom($dom);
echo $s->book[0]->title;
?>Поданий вище приклад виведе:
blah
Приклад #2 Importing a Dom\Document
<?php
$dom = Dom\XMLDocument::createFromString('<books><book><title>blah</title></book></books>');
$s = simplexml_import_dom($dom);
echo $s->book[0]->title;
?>Поданий вище приклад виведе:
blah
Why not just:
simplexml_import_dom(dom_import_simplexml($SimpleXmlNode))/*If tags in DOM have text and some part of this text is surrounded by other tag, simple_import_dom object doesn't contain position of child tag, so rendering it back to html/xml can be a problem. Printing such a node is also peculiar. Eg.*/
//////////////////////
$text = "<body><p>Some text before <em>italic </em>and after text.</p></body>";
$dom = new DOMDocument;
$dom->loadHTML($text);
$result = simplexml_import_dom($dom);
echo $result->p;
//////////////////////
//Prints :
//Some text before and after text.
print_r($result->p);
//Prints:
//SimpleXMLElement Object ( [em] => italic )
print_r($result->p->em)
//does more or less the same:
//SimpleXMLElement Object ( [0] => italic )
var_dump($result->p);
//Prints:
//object(SimpleXMLElement)#8 (1) { ["em"]=> string(7) "italic " }
var_dump($result->p->em)
//object(SimpleXMLElement)#9 (1) { [0]=> string(7) "italic " }
//but
echo $result->p->em;
//prints:
//italicsimplexml_import_dom() will accept DOMNodes or other SimpleXMLElements.  This has a useful purpose if you are extending the SimpleXMLElement class and would prefer the xpath() method to return your class extension instead of basic SimpleXMLElements.  For example:
class MySimpleXML extends SimpleXMLElement{
    public function xpath($xpath){ 
        $return = array();
        $simpleXmls = parent::xpath($xpath); // run SimpleXMLElement xpath, returning an array of SimpleXMLElements
        foreach($simpleXmls as $xml){
            $return[] = simplexml_import_dom($xml,'MySimpleXML'); // copy SimpleXMLElement, returning as MySimpleXML instance
        }
        return $return;
    }
}
Now I can create an instance of MySimpleXML, run an xpath, and get my results back as an array of MySimpleXML elements instead of the native SimpleXMLElement:
$mySimpleXml = new mySimpleXml('<root><node/></root>');
$array = $mySimpleXml->xpath('/root/node'); // returns array with a single MySimpleXml object