save form data with ajax json

Sorry for being so general but the fact is that i am really confused. I have tried doing it with xml with no success and now i am trying with json. I dont understand if i have to save the data as a string or in an array. I also saw a lot of times this code http_request.open("GET", url, true);, but i can't understand what kind of url i have to use.

Ok , when i said i tried xml, i did this but it didn't work:


<script>
function callServer() {
var xmlHttp;
try{
// Opera 8.0+, Firefox, Safari
xmlHttp = new XMLHttpRequest();
} catch (e){
// Internet Explorer Browsers
try{
xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
} catch (e) {
try{
xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
} catch (e){
// Something went wrong
alert("Your browser broke!");
return false;
}
}
}

var firstname = document.getElementById("firstname").value;
var lastname = document.getElementById("lastname").value;
var mail = document.getElementById("mail").value;
var mail2 = document.getElementById("mail2").value;
var username = document.getElementById("username").value;
var password = document.getElementById("password").value;
var password2 = document.getElementById("password2").value;

var xmlString = "<firstname>" + escape(firstname) +"" + "<lastname>" + escape(lastname) +"" +"<mail>" + escape(mail) + "" + " <mail2>" + escape(mail2) + "" + " <username>" + escape(username) + ""+ " <password>" + escape(password) + "</password>" + <password2> + escape(password2) + "</password2>";

//Build the URL to connect to
var url = "server.xml";
// Open a connection to the server
xmlHttp.open("GET", url, true);
// Tell the server you're sending it XML
xmlHttp.setRequestHeader("Content-Type", "text/xml");
// Set up a function for the server to run when it's done
// Send the request-
xmlHttp.send(xmlString);
}

function updatePage() {
if (xmlHttp.readyState == 4) {
if (xmlHttp.status == 200) {
var response = xmlHttp.responseText;
document.getElementById("firstname").value = response;
document.getElementById("lastname").value = response;
document.getElementById("mail").value = response;
document.getElementById("mail2").value = response;
document.getElementById("username").value = response;
document.getElementById("password").value = response;
document.getElementById("password2").value = response;
}
else if (xmlHttp.status == 404) {
alert("Request URL does not exist");
}
else {
alert("Error: status code is " + xmlHttp.status);
}
}
}
</script>


and after the previous code i tried this:

function loadJSON()
{
var http_request = new XMLHttpRequest();
try{
// Opera 8.0+, Firefox, Chrome, Safari
http_request = new XMLHttpRequest();
}catch (e){
// Internet Explorer Browsers
try{
http_request = new ActiveXObject("Msxml2.XMLHTTP");
}catch (e) {
try{
http_request = new ActiveXObject("Microsoft.XMLHTTP");
}catch (e){
// Something went wrong
alert("Your browser broke!");
return false;
}
}
}
http_request.onreadystatechange = function(){
if (http_request.readyState == 4 )
{
// Javascript function JSON.parse to parse JSON data
var jsonObj = JSON.parse(http_request.responseText);

// jsonObj variable now contains the data structure and can
// be accessed as jsonObj.name and jsonObj.country.
document.getElementById("username").innerHTML = jsonObj.firstname;
document.getElementById("password").innerHTML = jsonObj.lastname;
}
}
http_request.open("GET", "server.xml", true);
http_request.send();
}
</script>