Add two column in laravel

I want to add two column (pay_amount and discount) and put that result in another column. Controller for update function:

    $order['shipping_zip'] = $request->shipping_zip;
    $order['order_note'] = $request->order_note;
    $order['discount'] = $request->discount;

    $totalcost = $order['pay_amount'] - $order['discount'] ;
    $order['total_cost'] = $request->$totalcost;

    $order->save();

how can i do that is this one is right?

$totalcost = $order['pay_amount'] - $order['discount'] ;
    $order['total_cost'] = $request->$totalcost;