Validator::sometimes() method allows us to define when a validation rule should be applied

The laravel Validator::sometimes() method allows us to define when a validation rule should be applied, based on the input provided.

The snippet shows how to prohibit the use of a coupon if the quantity of the purchased items is not enough.

 1$data = [
 2    'coupon' => 'PIZZA_PARTY',
 3    'items' => [
 4        [
 5            'id' => 1,
 6            'quantity' => 2
 7        ],
 8        [
 9            'id' => 2,
10            'quantity' => 2,
11        ],
12    ],
13];
14 
15$validator = Validator::make($data, [
16    'coupon' => 'exists:coupons,name',
17    'items' => 'required|array',
18    'items.*.id' => 'required|int',
19    'items.*.quantity' => 'required|int',
20]);
21 
22$validator->sometimes('coupon', 'prohibited', function (Fluent $data) {
23    return collect($data->items)->sum('quantity') < 5;
24});
25 
26// throws a ValidationException as the quantity provided is not enough
27$validator->validate();

Tip given by @cerbero90