In theory you can provide the LIBXML_NOXMLDECL
option to drop the XML declaration when saving a document, but this is only available in Libxml >= 2.6.21 (and buggy). An alternative would be to use
$customXML = new SimpleXMLElement('<abc></abc>');
$dom = dom_import_simplexml($customXML);
echo $dom->ownerDocument->saveXML($dom->ownerDocument->documentElement);