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.

$data = [
    'coupon' => 'PIZZA_PARTY',
    'items' => [
        [
            'id' => 1,
            'quantity' => 2
        ],
        [
            'id' => 2,
            'quantity' => 2,
        ],
    ],
];
 
$validator = Validator::make($data, [
    'coupon' => 'exists:coupons,name',
    'items' => 'required|array',
    'items.*.id' => 'required|int',
    'items.*.quantity' => 'required|int',
]);
 
$validator->sometimes('coupon', 'prohibited', function (Fluent $data) {
    return collect($data->items)->sum('quantity') < 5;
});
 
// throws a ValidationException as the quantity provided is not enough
$validator->validate();

Tip given by @cerbero90