How to I insert new nodes into XML?

[PnzrDrgoon]

I'm trying to make an XML shopping cart that looks like:

<Cart>
     <Items>
     </Items>
</Cart>

 

I want to add an Item node to Items so it looks like:

<Cart>
     <Items>
         <Item>
              <SKU>1234</SKU>
              <Price>3.14</Price>
         </Item>
     </Items>
</Cart>

 

From what I've tried SimpleXML isn't cutting it (although I may have been trying to do it wrong) so I've tried DOMDocument:

--SNIP--

protected $cart="";
function __construct()
{
$empty_cart_xml=
			"<Cart>
				<Items>
				</Items>
			</Cart>";
	$this->cart=new DOMDocument;
	$this->cart->loadXML($empty_cart_xml);
}

public function add_item($sku,$merchant_id,$title,$amount,$curreny_code="USD",$quantity=1)
{		
	$node=new DOMNode();
	$item=$node->createElement("Item");
	$node->appendChild($item);

	$n_sku=$item->createElement("SKU",$sku);
	$item->appendChild($n_sku);

	$n_merchant_id=$item->createElement("MerchantID",$merchant_id);
	$item->appendChild($n_merchant_id);

	$n_title=$item->createElement("Title",$title);
	$item->appendChild($n_title);

	$n_price=$item->createElement("Price");
	$item->appendChild($n_price);

	$n_amount=$n_price->createElement('Amount',$amount);
	$n_price->appendChild($n_amount);

	$n_currency_code=$n_price->createElement('CurrencyCode',$curreny_code);
	$n_price->appendChild($n_currency_code);

	$n_quantity=$item->createElement("Quantity",$quantity);
	$item->appendChild($n_quantity);

	$this->cart->firstChild->firstChild->appendChild($node);
	echo $this->cart->saveXML();
}
--SNIP--

 

Can someone explain to me why the above doesn't do what I think it should be doing?