public function strength(string $password): int
    {
        $score = 0; // Initialize.

        if (!isset($password[0])) {
            return $score;
        }
        if (preg_match('/\p{N}/u', $password)) {
            $score += 25; // Number.
        }
        if (preg_match('/\p{Ll}/u', $password)) {
            $score += 25; // Lowercase.
        }
        if (preg_match('/\p{Lu}/u', $password)) {
            $score += 25; // Uppercase.
        }
        if (preg_match('/[^\p{N}\p{Ll}\p{Lu}]/u', $password)) {
            $score += 25; // Special symbol.
        }
        return $score;
    }