XML and Json

[i73]

Hey guys I am Posting data over to a server file in Json and I need to know how I can add to the XML file. here is both codes. 

function validate() {
	$xml = simplexml_load_file("stock.xml");
	$sxe = new SimpleXMLElement($xml->asXML());
	
	for($i = 1; $i < 801; $i++)
	{
		$newItem->addChild("name" , "null");
	}
	
	$sxe->asXML("stock.xml");
	echo $sxe->asXML();
	
    $response = $_POST;

    $response["items"] = "Test output";
    $response["description"] = "test 42 ";
    
    $response["json"] = json_encode($response);
    echo json_encode($response);
}

The red text is what I want this json script to do. Do I have to return back to my form to add to the XML file? Can json add to a xml file on its own? Thanks for your time! have a good day!