How to validate if an element of an array is an array itself?

I had the exact same problem two nights ago.

The conclusion at the very end was that Symfony2 validation has no "fast-fail" validation. That is, even if your Type() constraint would fail it would proceed with other constraints and thus fail with UnexpectedTypeException exception.

However, I was able to find a way to tackle that:

$constraint = new Collection([    'key' => new Required([        new Type(['type' => 'array']),        new Collection([            // Need to wrap fields into this            // in order to provide "groups"            'fields' => [                 'value' => new Required([                    new NotBlank(),                ]),            ],            'groups' => 'phase2' // <-- THIS IS CRITICAL        ]),    ]),]);// In your controller, service, etc...$V = $this->get('validator');// Checks everything by `Collection` marked with special group$violations = $V->validate($data, $constraint);if ( $violations->count()){    // Do something}// Checks *only* "phase2" group constraints$violations = $V->validate($data, $constraint, 'phase2');if ( $violations->count()){    // Do something}

Hope that this helps a bit. Personally, I find it annoying that we need to do this. Some sort of "fast-fail" flag within validator service would be much helpful.

You're saying the Collection constraint should just fail instead of throwing an exception because 'value' is a string and not an array.

There is a recently logged Symfony bug for this: https://github.com/symfony/symfony/issues/14943

Use Callback constraint(docs) where you can implement your custom validation logic.

The other way is to create custom constraint and validator classes. (docs)