xmlhttp.open (url) and Call PHP function using AJAX does not work?

Just beginner in php, I want to call php method using AJAX. I tried every thing but don't know what error is. Not getting any response from object xmlhttp.

Heres my java script code :

function loadData(){
    var mID=ddItems;
    var method=2;
    var xmlhttp;
    if (window.XMLHttpRequest) {
        xmlhttp = new XMLHttpRequest();
    }
    if (xmlhttp.readyState == 4 || xmlhttp.readyState == 0) {
        xmlhttp.open("GET", "../code/GetItemsInDD.class.php?id=" + mID + "&method=" + method, true); **// is this statement correct**

    xmlhttp.onreadystatechange = function(){
        if (xmlhttp.readyState==4 && xmlhttp.status==200) **//conditin is false,**
        {
            document.getElementById("ddItems").innerHTML=xmlhttp.responseText;
        }
     }
    xmlhttp.send();
    }
}

My js file is on "projectname/javascript/script.js" and my php file is in "projectname/code/GetItemsInDD.class.php" dir.

---

Why don't you use jQuery for making AJAX requests? Its as simple as this, include jQuery in your page

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.9.0/jquery.min.js"></script>

and JS code,

$.ajax({
  type: 'GET',
  url: '../code/GetItemsInDD.class.php?id=" + mID + "&method=" + method',
  success: function (data) {
     document.getElementById("ddItems").innerHTML = data;
  }
});

This way, you don't need to check for the readyState and status thing

jQuery follows object oriented approach for declaring XMLHttpRequest objects, so you won't have to worry about creating multiple objects for making more than one AJAX requests.