adding doctype in xml with php domdocument

In php I create my XML using the domdocument:

$xml_data = new DOMDocument("1.0", "UTF-8");

// create root element
$root = $xml_data->createElement("cXML");
$xml_data->appendChild($root);

etcetera

Is it possible to add this element:

<!DOCTYPE cXML
  SYSTEM 'http://xml.cXML.org/schemas/cXML/1.2.008/cXML.dtd'>

In to the XML with PHP domdocument, so you get:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE cXML
  SYSTEM 'http://xml.cXML.org/schemas/cXML/1.2.008/cXML.dtd'>