Description:
------------
When passing an object CustomDOMDocument extending DOMDocument as the DOMXPath object, the document property of DOMXpath is of type CustomDOMDocument.

As soon as the original document is destructed (because of scope), a clone of the object is stored internally, but with type DOMDocument and not the CustomDOMDocument class.


See https://3v4l.org/XLInV

Test script:
---------------
<?php


class CustomDOMDocument extends DOMDocument
{
}

function localScope()
{
    $doc = new CustomDOMDocument;
    $doc->loadHtml('<p>Test123</p>');
    $xpath = new DOMXPath($doc);
    
    return $xpath;
}


$doc = new CustomDOMDocument;
$doc->loadHtml('<p>Test123</p>');
$xpath = new DOMXPath($doc);

var_dump(get_class($xpath->document));


$xpath = new DOMXPath(new CustomDOMDocument);
var_dump(get_class($xpath->document));

$xpath = localScope();
var_dump(get_class($xpath->document));

Expected result:
----------------
string(17) "CustomDOMDocument"
string(17) "CustomDOMDocument"
string(17) "CustomDOMDocument"

Actual result:
--------------
string(17) "CustomDOMDocument"
string(11) "DOMDocument"
string(11) "DOMDocument"