A little experiment: Validating (potentially multi-leveled) arrays with the Symfony2 Validator component and returning the errors in the same data structure as the validated array by using the Symfony2 PropertyAccess component.

array-validation-error-mapping.php

<?php

use Symfony\Component\PropertyAccess\PropertyAccess;
use Symfony\Component\Validator\Constraints\All;
use Symfony\Component\Validator\Constraints\Choice;
use Symfony\Component\Validator\Constraints\Collection;
use Symfony\Component\Validator\Constraints\Length;
use Symfony\Component\Validator\Constraints\NotBlank;
use Symfony\Component\Validator\Constraints\Optional;
use Symfony\Component\Validator\Constraints\Required;
use Symfony\Component\Validator\Constraints\Type;
use Symfony\Component\Validator\ConstraintViolationInterface;
use Symfony\Component\Validator\Validation;

require_once __DIR__.'/vendor/autoload.php';

$validator = Validation::createValidator();
$propertyAccessor = PropertyAccess::createPropertyAccessor();

$array = array(
    'firstName' => 'Be',
    'hobbies' => array(
        array(
            'name' => 'Hiking',
            'frequency' => 'monthly',
        ),
        array(
            'name' => 'Cooking',
        ),
    )
);

// Validate values
$constraint = new Collection(array(
    'firstName' => new Required(array(
        new NotBlank(),
        new Length(array('min' => 3)),
    )),
    'lastName' => new Required(array(
        new NotBlank(),
        new Length(array('min' => 3)),
    )),
    'hobbies' => new Optional(array(
        new Type('array'),
        new All(
            new Collection(array(
                'name' => new Required(array(
                    new NotBlank(),
                    new Length(array('min' => 3)),
                )),
                'frequency' => new Required(array(
                    new Choice(array('daily', 'weekly', 'monthly')),
                ))
            ))
        ),
    )),
));

$violations = $validator->validateValue($array, $constraint);

// Use the same structure for the errors
$errors = array();

foreach ($violations as $violation) {
    /** @var ConstraintViolationInterface $violation */
    $entryErrors = (array) $propertyAccessor->getValue($errors, $violation->getPropertyPath());
    $entryErrors[] = $violation->getMessage();

    $propertyAccessor->setValue($errors, $violation->getPropertyPath(), $entryErrors);
}

var_export($errors);
echo "\n";

output

array (
  'firstName' => 
  array (
    0 => 'This value is too short. It should have 3 characters or more.',
  ),
  'lastName' => 
  array (
    0 => 'This field is missing.',
  ),
  'hobbies' => 
  array (
    1 => 
    array (
      'frequency' => 
      array (
        0 => 'This field is missing.',
      ),
    ),
  ),
)