Particle\Validator is a very small validation library, with the easiest and most usable API we could possibly create.
$v = new Validator;
$v->required('first_name')->length(5);
$v->validate(['first_name' => 'Berry']); // bool(true).
$v->required('last_name')->length(10);
$v->validate(['first_name' => 'Berry']); // bool(false).
$v->getMessages(); // array with error messages.