multiple users for shopping cart - cart

Question

currently I'm doing a bookstore system. almost similar with another shopping cart like amazon or whatsoever. my problem is how to apply multiple users to add product on cart . i do have login session and i have set MM_username for each user login. but when users add product into cart, it only appears name of one user.
here's the code below
if(isset($_POST["place_order"]))
{
$insert_order = "
INSERT INTO tbl_order(customer_id, creation_date, order_status)
VALUES(103, now(), 'pending')
";
$order_id = "";
if(mysqli_query($connect, $insert_order))
{
$order_id = mysqli_insert_id($connect);
}
$_SESSION["order_id"] = $order_id;
$order_details = "";
foreach($_SESSION["shopping_cart"] as $keys => $values)
{
$order_details .= "
INSERT INTO tbl_order_details(order_id, product_name, product_price, product_quantity)
VALUES('".$order_id."', '".$values["product_name"]."', '".$values["product_price"]."', '".$values["product_quantity"]."');
";
}
if(mysqli_multi_query($connect, $order_details))
{
unset($_SESSION["shopping_cart"]);
echo '<script>alert("You have place an order...your order status will be inform through phone call within maximum 3 days..Thank you")</script>';
echo '<script>window.location.href="cart.php"</script>';
}
}
if(isset($_SESSION["order_id"]))
{
$customer_details = '';
$order_details = '';
$total = 0;
$query = '
SELECT * FROM tbl_order
INNER JOIN tbl_order_details
ON tbl_order_details.order_id = tbl_order.order_id
INNER JOIN tbl_customer
ON tbl_customer.CustomerID = tbl_order.customer_id
WHERE tbl_order.order_id = "'.$_SESSION["order_id"].'"
';
$result = mysqli_query($connect, $query);
while($row = mysqli_fetch_array($result))
{
$customer_details = '
<label>'.$row["CustomerName"].'</label>
<p>'.$row["Address"].'</p>
<p>'.$row["City"].', '.$row["PostalCode"].'</p>
<p>'.$row["Country"].'</p>
';
$order_details .= "
<tr>
<td>".$row["product_name"]."</td>
<td>".$row["product_quantity"]."</td>
<td>".$row["product_price"]."</td>
<td>".number_format($row["product_quantity"] * $row["product_price"], 2)."</td>
</tr>
";
$total = $total + ($row["product_quantity"] * $row["product_price"]);
}
echo '
<h3 align="center">Order Summary for Order No.'.$_SESSION["order_id"].'</h3>
<div class="table-responsive">
<table class="table">
<tr>
<td><label>Customer Details</label></td>
</tr>
<tr>
<td>'.$customer_details.'</td>
</tr>
<tr>
<td><label>Order Details</label></td>
</tr>
<tr>
<td>
<table class="table table-bordered">
<tr>
<th width="50%">Product Name</th>
<th width="15%">Quantity</th>
<th width="15%">Price</th>
<th width="20%">Total</th>
</tr>
'.$order_details.'
<tr>
<td colspan="3" align="right"><label>Total</label></td>
<td>'.number_format($total, 2).'</td>
</tr>
</table>
</td>
yes i know i shouldn't have put 103 as customer_id and i know that is the reason it will only read 103 user, but i have tried many ways include filter MM_username, ' ', $value and whatsoever. but it didn't work.