php ajax用户登录代码

function chk_login(form){
    var username=form.username;
    var password=form.password;
    if(username.value == ""){
        alert("用户名不能为空");
        username.focus();
        return false;
    }
    if(password.value == ""){
        alert("密码不能为空");
        password.focus();
        return false;
    }

    var url = 'login_chk.php?username='+username.value+'&password='+password.value;
    xmlhttp.open('get',url,true);
//为什么下面加了下面注释掉的代码后，会没反应了，没加就有反应，我的php文件路径都是正确的，也有引入xmlhttprequest.js
xmlhttp.onreadystatechange = function(){
        if(xmlhttp.readyState == 4){
            if(xmlhttp.status == 200){
                var msg = xmlhttp.responseText;
                if(msg == "1"){
                    alert("登陆成功");
                    window.location='index.php?name='+username.value;
                }else if((msg == "2"){
                    alert("登陆用户名或密码错误");
                }else{
                    alert(msg);
                }
            }
        }
    }
    xmlhttp.send(null);*/
}

ogin