Can You find the syntax error?

Hello, I’ve to create a function where I’ve to input a custom field with the name (NumDdt and DateDdt), but this function doesn’t work:
function modifica_fattura_elettronica($result_array, $order ) {
$importo = (float)$order->get_total();
$data = $order->get_date_created();
$NumDdt = $order->custom_field(‘NumDdt’);
$DateDdt = $order->custom_field(‘DateDdt’);

$doc = new DOMDocument();
$doc->preserveWhiteSpace = false;
$doc->formatOutput = true;
$doc->loadXML($result_array[“xml”]);
$FatturaElettronicaBody = $doc->getElementsByTagName(“FatturaElettronicaBody”)->item(0);

$DatiDDT = $doc->createElement(“DatiDDT”);
$DatiTrasporto = $doc->createElement($NumDdt); //TP02 = pagamento completo
$DatiDDT->appendChild($DatiTrasporto);

$FatturaElettronicaBody->appendChild($DatiDDT);

$xml = $doc->saveXML();
$result_array[“xml”] = $xml;
return $result_array;
}
add_action( ‘wcpdf_IT_after_creaFatturaElettronica’, ‘modifica_fattura_elettronica’, 20, 2 );

Anyone can help me? thank you