How to Validate Raw Values (Scalar Values and Arrays)

Usually you will be validating entire objects. But sometimes, you just want to validate a simple value - like to verify that a string is a valid email address. From inside a controller, it looks like this:

// ...
use Symfony\Component\Validator\Constraints as Assert;
use Symfony\Component\Validator\Validator\ValidatorInterface;

// ...
public function addEmail($email, ValidatorInterface $validator)
{
    $emailConstraint = new Assert\Email();
    // all constraint "options" can be set this way
    $emailConstraint->message = 'Invalid email address';

    // use the validator to validate the value
    $errors = $validator->validate(
        $email,
        $emailConstraint
    );

    if (0 === count($errors)) {
        // ... this IS a valid email address, do something
    } else {
        // this is *not* a valid email address
        $errorMessage = $errors[0]->getMessage();

        // ... do something with the error
    }

    // ...
}

By calling validate() on the validator, you can pass in a raw value and the constraint object that you want to validate that value against. A full list of the available constraints - as well as the full class name for each constraint - is available in the constraints reference section.

Validation of arrays is possible using the Collection constraint:

use Symfony\Component\Validator\Constraints as Assert;
use Symfony\Component\Validator\Validation;

$validator = Validation::createValidator();

$input = [
    'name' => [
        'first_name' => 'Fabien',
        'last_name' => 'Potencier',
    ],
    'email' => 'test@email.tld',
    'simple' => 'hello',
    'eye_color' => 3,
    'file' => null,
    'password' => 'test',
    'tags' => [
        [
            'slug' => 'symfony_doc',
            'label' => 'symfony doc',
        ],
    ],
];

$groups = new Assert\GroupSequence(['Default', 'custom']);

$constraint = new Assert\Collection([
    // the keys correspond to the keys in the input array
    'name' => new Assert\Collection([
        'first_name' => new Assert\Length(['min' => 101]),
        'last_name' => new Assert\Length(['min' => 1]),
    ]),
    'email' => new Assert\Email(),
    'simple' => new Assert\Length(['min' => 102]),
    'eye_color' => new Assert\Choice([3, 4]),
    'file' => new Assert\File(),
    'password' => new Assert\Length(['min' => 60]),
    'tags' => new Assert\Optional([
        new Assert\Type('array'),
        new Assert\Count(['min' => 1]),
        new Assert\All([
            new Assert\Collection([
                'slug' => [
                    new Assert\NotBlank(),
                    new Assert\Type(['type' => 'string'])
                ],
                'label' => [
                    new Assert\NotBlank(),
                ],
            ]),
            new CustomUniqueTagValidator(['groups' => 'custom']),
        ]),
    ]),
]);

$violations = $validator->validate($input, $constraint, $groups);

The validate() method returns a ConstraintViolationList object, which acts just like an array of errors. Each error in the collection is a ConstraintViolation object, which holds the error message on its getMessage() method.