最简单的用户验证功能 Ajax菜鸟 不知道以上代码错哪了 求大神解救！

// JavaScript Document

//var xmlHttp;   

    function createXMLHttpRequest()   

    {   

          if(window.XMLHttpRequest)   

        {   

             var xmlHttp = new XMLHttpRequest();//mozilla浏览器   

        }   

        else if(window.ActiveXObject)   

        {   

            try  

            {   

            var xmlHttp = new ActiveX0bject("Msxml2.XMLHTTP");//IE老版本   

            }   

            catch(e)   

            {}   

            try  

            {   

               var xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");//IE新版本   

            }   

            catch(e)   

            {}   

            if(!xmlHttp)   

            {   

                window.alert("不能创建XMLHttpRequest对象实例");   

                return false;   

            }   

			else

		 	 return xmlHttp; 

        }   

    }   

       

       

   function startRequest(username)   

        {   

              

            createXMLHttpRequest(); 

               

              xmlHttp.open("GET","checkname.php?name="+username,true);   

            xmlHttp.onreadystatechange = handleStateChange;   

            xmlHttp.send(null);   

        }