<form id="myform" name="myform" action="03.php" method="post">

        <table>

            <tr>

                <td>用户名：</td>

                <td><input type="text" id="user" name="user"></td>

            </tr>

            <tr>

                <td>密码：</td>

                <td><input type="password" id="pwd" name="pwd"></td>

            </tr>

            <tr>

                <td></td>

                <td><input type="button" id="btn" value="登录"></td>

            </tr>

        </table>

    </form>

//使用Ajax时候不要用subbmit

    var btn=document.getElementById("btn");

    btn.onclick=function(){

        var xhr=getXhr();

        xhr.open('post','03.php');

        xhr.setRequestHeader("Content-Type","application/x-www-form-urlencoded");

        var user=document.getElementById("user").value;

        var pwd=document.getElementById("pwd").value;

        xhr.send("user="+user+"&pwd="+pwd);

        xhr.onreadystatechange=function(){

        //获得服务器端当前状态

        //alert(xhr.readyState);

        //保证响应完成

            if(xhr.readyState==4){

                //保证这次请求必须成功

                //alert(xhr.status);

                if(xhr.status==200){

                    //接收服务器端的数据

                    var data=xhr.responseText;

                    //测试

                    console.log(data);

                    }

                }

        }

    };

    function getXhr(){

        var xhr=null;

        if(window.XMLHttpRequest){

            xhr=new XMLHttpRequest();

        }else{

            xhr=new ActiveXObject("Microsoft.XMLHttp");

        }

        return xhr;

    }

<?php

    $user=$_POST['user'];

    $pwd=$_POST['pwd'];

    if($user=="zhouzhiruo"&$pwd==

        "123456"){

        echo "login successful";

    }else{

        echo "login error";

    }

?>