open（POST）ajax

Question

当我在php文件中调用它时，我有这个参数的问题 显示此错误：未定义的索引：qu，class，sub

function sender(qu , sub)
{
    var classs = getCheckedRadioValue('classs');`enter code here`
    var xmlhttp = new XMLHttpRequest();
    xmlhttp.onreadystatechange = function(){
        if(xmlhttp.readyState == 4 && xmlhttp.status == 200){
            return true;
        }
    }
    xmlhttp.open("POST" , "controller/add_qu.php?qu="+qu+"&class="+classs+"&sub="+sub , true);
    xmlhttp.send();
}