$paypal_product['items'][] = array(.....);

foreach($_POST['item_name'] as  $key=>$itmname)
{
    //create items for session
    $paypal_product['items'][] = array(

      'itm_name'=>$_POST["item_name"][$key],
      'itm_price'=>$_POST["amount"][$key],
      'itm_code'=>$_POST['item_code'][$key],
      'itm_qty'=>$_POST['quantity'][$key],
      'itm_type'=>$_POST['productType'][$key],
      'itm_desc'=>$_POST['description'][$key]
                                        );
}

echo "Product Code: " . $paypal_product['itm_code'][0];