(PHP 8 >= 8.4.0)
    Each Dom\Document has a doctype
    attribute whose value is either null or a Dom\DocumentType object.
   
This is the modern, spec-compliant equivalent of DOMImplementation.
The public identifier of the external subset.
The system identifier of the external subset. This may be an absolute URI or not.
       The name of DTD; i.e., the name immediately following the
       DOCTYPE keyword.
      
       The internal subset as a string, or null if there is none. This
       does not contain the delimiting square brackets.
      
