XML response

header("Content-Type: text/xml");
$sum = $_GET["figure1"] + $_GET["figure2"];
echo <<< END_OF_FILE

{$_GET["figure1"]}

{$_GET["figure2"]}

END_OF_FILE

?>

The callback function now extracts the result using responseXML. It then has a first-class DOM object and can interrogate it using the standard DOM API.

var xhReq = createXMLHttpRequest();
xhReq.open("GET", "sumXML.phtml?figure1=10&figure2=20", true);
xhReq.onreadystatechange = function() {

if (xhReq.readyState != 4) { return; }
** xml = xhReq.responseXML;
var figure1 = xml.getElementsByTagName("figure")[0].firstChild.nodeValue;
var figure2 = xml.getElementsByTagName("figure")[1].firstChild.nodeValue;
var sum = xml.getElementsByTagName("outputs")[0].firstChild.nodeValue; **
...
};
xhReq.send(null);
});


The name "XMLHttpRequest" relates to its two primary functions: handling HTTP requests and converting XML responses. The former function is critical and the latter is best considered a bonus. There are certainly good applications for XML responses - see XML_Message, XML_Data_Island, and Browser-Side_XSLT - but keep in mind that XML is not a requirement of Ajax systems.
You can also upload XML. In this case, XMLHttpRequest doesn't offer any special XML functionality; you just send the XML message as you would any other message, and with an appropriate request type (e.g. "POST" or "PUT"). For the sake of the receiving web service, the Javascript should generally declare the XML content type in a request header. xhReq.setRequestHeader('Content-Type', "text/xml");