function createUser(array $data) {
    if (empty($data['email'])) {
        throw new InvalidArgumentException("Email is required");
    }

    if (!filter_var($data['email'], FILTER_VALIDATE_EMAIL)) {
        throw new InvalidArgumentException("Invalid email format");
    }

    if (emailExists($data['email'])) {
        throw new RuntimeException("Email already taken");
    }

    return saveUser($data);
}

function createUser(
    array $data,
    callable $emailExists,
    callable $saveUser
): int {
    $email = $data['email'] ?? null;

    if ($email === null || trim($email) === '') {
        throw new InvalidArgumentException('Email is required');
    }

    if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
        throw new InvalidArgumentException('Invalid email format');
    }

    if ($emailExists($email)) {
        throw new DomainException('Email already taken');
    }

    return $saveUser($data);
}