Value of $total not being set inside lumen collection map function

I have a lumen collection object for my order and use map function to iterate over it and do some logic. I also need the total value of the order which is calculated using quantity * price. But the variable that is responsible for holding the total value is always 0.

$total = 0;
$items = Cart_Item::where('user_id', $user->id)->get();
$items = $items->map(function ($item, $key) use ($order, $total) {
    $product = Product::where('id', $item->product_id)->first();
    $order_item = new Order_Item();
    $order_item->order_id = $order->id;
    $order_item->product_id = $item->product_id;
    $order_item->quantity = $item->quantity;
    $order_item->price = $product->GetPrice->price;
    $order_item->save();
    $total = $total + ($order_item->quantity * $order_item->price);
});

No matter what I do, the $total always returns 0.

The scope of your closure is not that of the entire file, but it's limited only to what's between the {} tags.

The used variables are copied into the function scope.

One solution would be to make $total a global variable:

$total = 0;
$items = Cart_Item::where('user_id', $user->id)->get();
$items = $items->map(function ($item, $key) use ($order, $total) {
    $product = Product::where('id', $item->product_id)->first();
    $order_item = new Order_Item();
    $order_item->order_id = $order->id;
    $order_item->product_id = $item->product_id;
    $order_item->quantity = $item->quantity;
    $order_item->price = $product->GetPrice->price;
    $order_item->save();

    global $total;
    $total = $total + ($order_item->quantity * $order_item->price);
});

Another is to pass global as a reference &$total like so:

$total = 0;
$items = Cart_Item::where('user_id', $user->id)->get();
$items = $items->map(function ($item, $key) use ($order, &$total) {
    $product = Product::where('id', $item->product_id)->first();
    $order_item = new Order_Item();
    $order_item->order_id = $order->id;
    $order_item->product_id = $item->product_id;
    $order_item->quantity = $item->quantity;
    $order_item->price = $product->GetPrice->price;
    $order_item->save();

    $total = $total + ($order_item->quantity * $order_item->price);
});