使用 ajax 将 JSON 发送到 PHP

我想将一些json格式的数据发送到php，并在php中执行一些操作.我的问题是我无法通过ajax将json数据发送到我的php文件.请帮帮我怎么做.我试过这种方法..

<script>
$(function (){
 $("#add-cart").click(function(){
    var bid=$('#bid').val();
    var myqty=new Array()
    var myprice=new Array()

    qty1=$('#qty10').val();
    qty2=$('#qty11').val();
    qty3=$('#qty12').val();

    price1=$('#price1').val();
    price2=$('#price2').val();
    price3=$('#price3').val();

    var postData = 
                {
                    "bid":bid,
                    "location1":"1","quantity1":qty1,"price1":price1,
                    "location2":"2","quantity2":qty2,"price2":price2,
                    "location3":"3","quantity3":qty3,"price3":price3
                }
    var dataString = JSON.stringify(postData);

    $.ajax({
            type: "POST",
            dataType: "json",
            url: "add_cart.php",
            data: {myData:dataString},
            contentType: "application/json; charset=utf-8",
            success: function(data){
                alert('Items added');
            },
            error: function(e){
                console.log(e.message);
            }
    });
});
});
</script>

在PHP中，我使用:

if(isset($_POST['myData'])){
 $obj = json_decode($_POST['myData']);
 //some php operation
}

When in add print_r($_POST) in php file, it shows array(0) {} in firebug.