Laravel - Validation - Using Closures

If you only need the functionality of a custom rule once throughout your application, you may use a closure instead of a rule object. The closure receives the attribute's name, the attribute's value, and a $fail callback that should be called if validation fails:

    use Illuminate\Support\Facades\Validator;
    $validator = Validator::make($request->all(), [
        'title' => [
            function ($attribute, $value, $fail) {
                if ($value === 'foo') {
                    $fail('The '.$attribute.' is invalid.');