GrumPHP\Linter\Xml\XmlLinter::validateDTD PHP Method

    private function validateDTD(DOMDocument $document)
    {
        if (is_null($document->doctype)) {
            return true;
        }
        // Do not validate external DTDs if the loadFromNet option is disabled:
        $systemId = $document->doctype->systemId;
        if (filter_var($systemId, FILTER_VALIDATE_URL) && !$this->loadFromNet) {
            return true;
        }
        return $document->validate();
    }