$validator = new Validation\ArrayValidator();

$validator->addConstraint('first_name', new Validation\Assert\Length(['min' => 3]));

$validator->addConstraint('last_name', new Validation\Assert\Present());

$violations = $validator->validate(['first_name' => 'me']);

echo $violations->count(); // 1
echo count($violations); // 1

print_r($violations->getMessages());

Array
(
    [first_name] => Array
        (
            [0] => first name must be greater than or equal to 3 characters
        )

    [last_name] => Array
        (
            [0] => last name must be present
        )
)