function getXmlHttpRequestObject() {
 if (window.XMLHttpRequest) {
    return new XMLHttpRequest();
 } else if (window.ActiveXObject) {
    return new ActiveXObject("Microsoft.XMLHTTP");
 } else {
    alert("Your browser doesn't support the XmlHttpRequest object.");
 }
}
 
var receiveReq = getXmlHttpRequestObject();
var form = document.getElementById('frm');
//var response = document.getElementById('response');
 
//Initiate the AJAX request
function makeRequest(url, param) {
//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 = updatePage;
 
   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() {
 //Check if our response is ready
 if (receiveReq.readyState == 4) {
     if (receiveReq.responseText==="no") {
   //Set the content of the DIV element with the response text
   //document.getElementById('response').innerHTML = receiveReq.responseText;
   $("#respose").addClass('messageboxerror').text('Введенные символы должны совпадать с символами на картинке !');
   //Get a reference to CAPTCHA image
   img = document.getElementById('img');
   //Change the image
   img.src = '/img.php?' + Math.random(); return false;} else {form.submit();}
 }
}
 
//Called every time when form is perfomed
function getParam(theForm) {
 //Set the URL
 var url = '/captch.php';
 //Set up the parameters of our AJAX call
 var postStr = theForm.tx.name + "=" + encodeURIComponent( theForm.tx.value );
 //Call the function that initiate the AJAX request
 makeRequest(url, postStr);
}

onSubmit="getParam(this)"