登录页面ajax.

    var xmlHttp = CreateXmlHttp();
    xmlHttp.open("post", "../login.php?act=login", true);
    xmlHttp.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
    $('shop_login').style.display = 'none';
    $('shop_info').style.display = 'none';
    $('loading').style.display = 'block';
    xmlHttp.onreadystatechange = function() {
        if (xmlHttp.readyState == 4) {
            if (xmlHttp.status == 200) {
                try {
                    var jsonData = eval('(' + xmlHttp.responseText + ')');
                    if (jsonData.result == 'ok') {
                        $("unk").innerHTML = $("unk").title = jsonData.unk;
                        $("uPig").innerHTML = $("uPig").title = jsonData.uPig;
                        $("uCash").innerHTML = $("uCash").title = jsonData.uCash;
                        $("uDen").innerHTML = $("uDen").title = jsonData.uDen;
                        $("uOnline").innerHTML = $("uOnline").title = jsonData.uOnline;
                        $("uFlower").innerHTML = $("uFlower").title = jsonData.uFlower;
                        $("uPay").innerHTML = $("uPay").title = jsonData.uPay;
                        $('loading').style.display = 'none';
                        $('shop_login').style.display = 'none';
                        $('shop_info').style.display = 'block';
                        $("iframeLoad").src = "goods.php"
                    } else {
                        $(ErrArr[jsonData.result].id).style.backgroundColor = ErrColor;
                        $('shop_info').style.display = 'none';
                        $('loading').style.display = 'none';
                        $('shop_login').style.display = 'block';
                        alert(ErrArr[jsonData.result].Msg);
                        $(ErrArr[jsonData.result].id).select()
                    }
                } catch(e) {
                    alert(xmlHttp.responseText)
                }
            } else {
                showTips(3)
            }
        }
    };
    xmlHttp.send('userid=' + userid.value + '&password=' + password.value + '&code=' + code.value)
}php页面        <div id="login">
            <div class="login_t"></div>
            <div class="login_l"></div>
            <div class="login_warp">
              <ul id="shop_info" style="display:none">
                <li>欢迎您，<strong id="unk"></strong>
                 <a href="javascript:;" onClick="javascript:loginout();" class="login_out">[退出]</a></li>
                <li><span>消费：<strong id="uCash" title=""></strong></span>
                <span>金钱：<strong id="uPig" title=""></strong></span></li>
                <li><span>元宝：<strong id="uDen" title=""></strong></span>
                <span>时长：<strong id="uOnline" title=""></strong></span></li>
                <li><span>声望：<strong id="uFlower" title=""></strong></span>
                <span>积分：<strong id="uPay" title=""></strong></span></li>
                <li><p id="welcome">欢迎进入</p></li>
              </ul>
              <ul id="shop_login" style="display:block">
                <form method="post" onSubmit="javascript:chkdata();return false;"  action="" >
                <li><span>账号：</span><input type="text" id="userid" maxLength="20"  /></li>
                <li><span>密码：</span><input type="password" id="password" maxLength="20" /></li>
                <li><span>验证码：</span>
                <input type="text" id="code" name="code" style="width:35px;" maxlength="4"/>
                <img src="../inc/captcha.php" id="codeform" onClick="updatacode(this,'../inc/captcha.php');" title="点击刷新"/>
                <a href="javascript:void(0);">忘记密码？</a></li>
                <li style="padding:5px 0px 0px 50px; height:35px;">
                <input class="btn" type="submit" value="登 录" />
                <input type="button" onclick="javascript:location.href='../reg.php'" class="btn" value="注 册" />
                </li>
                </form>
              </ul>
              <div id="loading" style="display:none"><p class="loading"><img src="../images/loading.gif" align="absmiddle" />正在登录，请稍等……</p></div>
            </div>
            <div class="login_r"></div>
            <div class="login_b"></div>
          </div>现在出现一个问题,登录后,有显示用户信息,但是如果刷新下,就变成未登录.请高手指教.