callback Checks with the callback sent by the user It's ideal for one-time custom validations kirby/src/Toolkit/V.php#L290 V::callback($value, callable $callback): bool Parameters Name Type Default $value * mixed – $callback * callable – Return type bool In your code V::callback( ['green', 'blue'], fn ($value) => count($value) === count(array_intersect($value, ['red', 'green', 'blue'])) );