ajax data were in xml,how in php parse/process them like prior

[leonidassavvides]

I know how to get
registerRequest.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
DATA from a request and enter them in database or response(echo) via ajax/php, well what if that ajax data were in xml (Basic) format, how in php parse/process them like prior?
give URLs...

function registerUser() {
  t = setInterval("scrollImages()", 50);
  document.getElementById("register").value = "Processing...";
  registerRequest = createRequest();
  if (registerRequest == null) {
    alert("Unable to create request.");
  } else {
    var url = "register-feedback.php";
    var requestData = "username=" + 
      escape(document.getElementById("username").value) + "&password=" +
      escape(document.getElementById("password1").value) + "&firstname=" +
      escape(document.getElementById("firstname").value) + "&lastname=" +
      escape(document.getElementById("lastname").value) + "&email=" +
      escape(document.getElementById("email").value) + "&genre=" +
      escape(document.getElementById("genre").value) + "&favorite=" +
      escape(document.getElementById("favorite").value) + "&tastes=" +
      escape(document.getElementById("tastes").value);
    registerRequest.onreadystatechange = registrationProcessed;
    registerRequest.open("POST", url, true);
    registerRequest.setRequestHeader("Content-Type",
      "application/x-www-form-urlencoded");
    registerRequest.send(requestData);
  }
}

function registrationProcessed() {
  if (registerRequest.readyState == 4) {
    if (registerRequest.status == 200) {
      document.getElementById('wrapper').innerHTML =
        registerRequest.responseText;
    }
  }
}

[leonidassavvides]

use
var doc = xhr.responseXML;
and parse xml dom tree?