//Gets the browser specific XmlHttpRequest Object
function getXmlHttpRequestObject()
{
 if (window.XMLHttpRequest) {
    return new XMLHttpRequest(); //Mozilla, Safari ...
 } else if (window.ActiveXObject) {
    return new ActiveXObject("Microsoft.XMLHTTP"); //IE
 } else {
    //Display our error message
    alert("Your browser doesn't support the XmlHttpRequest object.");
 }
}

//Our XmlHttpRequest object
var receiveReq = getXmlHttpRequestObject();

//Initiate the AJAX request
function makeRequest(url, param, theForm)
{
 //If our readystate is either not started or finished, initiate a new request
 if (receiveReq.readyState == 4 || receiveReq.readyState == 0)
 {
   //Set up the connection to captcha_test.html. True sets the request to asyncronous(default) 
   receiveReq.open("POST", url, true);
   //Set the function that will be called when the XmlHttpRequest objects state changes
   receiveReq.onreadystatechange = function()
	{
		/** Ora THIS e' questa funzione! **/
		if (receiveReq.readyState == 4) // The 4 state means for the response is ready and sent by the server.
		{
			if (receiveReq.status == 200) // This status means "OK", otherwise some error code is returned, 404 for example.
			{
				updatePage(theForm);
			}
		}
	}

   //Add HTTP headers to the request
   receiveReq.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
   receiveReq.setRequestHeader("Content-length", param.length);
   receiveReq.setRequestHeader("Connection", "close");

   //Make the request
   receiveReq.send(param);
 }
}

//Called every time our XmlHttpRequest objects state changes
function updatePage(theForm) {
   //Set the content of the DIV element with the response text
	if (receiveReq.responseText=="OK") {
		checkForm(theForm);
	}
	else
	{
		alert('The security code is wrong. Please try again.');
		document.getElementById('txtCaptcha').focus();
	   //Get a reference to CAPTCHA image
	   img = document.getElementById('imgCaptcha'); 
	   //Change the image
	   img.src = 'createCaptcha.php?' + Math.random();
	}
}

//Called every time when form is perfomed
function getParam(theForm)
{
 //Set the URL
 var url = 'checkCaptcha.php';
 //Set up the parameters of our AJAX call
 var postStr = "txtCaptcha=" + encodeURIComponent( document.getElementById("txtCaptcha").value );
 //Call the function that initiate the AJAX request
 makeRequest(url, postStr, theForm);
}