Description:
------------
---
From manual page: https://php.net/class.domdocumenttype
---

According to the `DOMNode` documentation [1], the `$childNodes` property always refers to a `DOMNodeList`.

However, `DOMDocumentType` extends `DOMNode`, but doesn't provide the property nor supports `hasChildNodes()`.

This means that the following test script causes a Notice.

[1] https://www.php.net/class.domnode.php

Test script:
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$dom = new DOMImplementation();
$type = $dom->createDocumentType('html');
echo get_class($type->childNodes);


Expected result:
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DOMNodeList

Actual result:
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Warning: get_class() expects parameter 1 to be object, null given in test.php on line 5

Call Stack:
    0.0075     398976   1. {main}() test.php:0