jquery ajax 跨域调用

js端：
$.ajax({
            type : "GET",
            url : "http://****.com/***.php?callback=?",
            dataType : "jsonp",
            jsonp: 'callback',
            success : function(json){
                alert(json.msg);
               
            }
        });
服务端:
$callback = $_GET['callback'];
echo "{$callback}({'msg':'this is a jquery jsonp test message!'})";
exit();