PHP与JavaScript间的数据传递

<script type="text/javascript">
function GetJson() 
{
    var xmlHttp;
    try
    {
        // Firefox, Opera 8.0+, Safari
        xmlHttp = new XMLHttpRequest();
    }
    catch (e) 
    {
        // Internet Explorer
        try {
            xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
        }
        catch (e) 
        {
            try {
                xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
            }
            catch (e) {
                alert("您的浏览器不支持AJAX！");
                    return false;
            }
        }
    }
     
    xmlHttp.onreadystatechange = function() 
    {
        if (xmlHttp.readyState == 4) 
        {
            //alert(xmlHttp.responseText);
            var str = xmlHttp.responseText;
            document.getElementById('show').innerHTML +=str;
            //alert(str);
            var obj = eval('('+ xmlHttp.responseText +')');
            //var obj = eval(({"id":"123","name":"elar","age":"21"}));
            alert(obj.name);
        }
    }
    var data = "id=123";
    xmlHttp.open("POST", "testJson.php", true);
    xmlHttp.setRequestHeader("Content-Type","application/x-www-form-urlencoded"); 
    xmlHttp.send("id=123");
}
</script>
<input type="button" onclick="GetJson()" value="按我！"/>
<hr />
<div id="show"></div>

<?php 
$res['id'] = $_POST['id'];
$res['name'] = "elar";
$res['age'] = "21";
$response = "hello this is response".$_POST['id'];
echo json_encode($res);
?>