PDO插入问题

I'm trying to pass an object using jQuery post. It passes alright but something goes wrong with the php code:

} elseif($_GET['action'] == "insertQty") {
    $obj = json_decode($_POST['param']);
    $scArr = $obj->{'data'};
    $od_id = $obj->{'id'};

    for($i = 0; $i < count($scArr); $i++){
        $pd_id = substr($scArr[$i], 5);
        $pQty = $obj->{$scArr[$i]};

        $stmt = $DBH->prepare("INSERT INTO tbl_order_item_test (od_id, pd_id, od_qty) VALUES (:oId, :pId, :pQty)");

        $stmt->bindParam(':oId', $od_id);
        $stmt->bindParam(':pId', $pd_id);
        $stmt->bindParam(':pQty', $pQty);

        $stmt->execute(); 
    }
}

$scArr[$i] = "prod_#" where # is some number. Therefore substr(5) = #.

the obj:

var array = getArray();  
var itemsObj = {};

itemsObj['data'] = shoppingCart;
itemsObj['id'] = data;

//for-loop here to dynamically insert data to the obj.

$.post(url, "param=" + JSON.stringify(itemsObj), "html");

What could not be working properly? It's most likely a problem with the first paragraph of code since the object comes out as it should be when using alert().

EDIT:

print_r($obj):

stdClass Object
(
     [data] => Array
        (
             [0] => prod_2
        )

     [id] => 1024
     [prod_2] => 8
)

You can pretty much guess the outpud of $scArr from the above... Basically everything object-related is fine.