public function isPrime()
    {
        if ($this->value < 2) {
            return false;
        }

        if ($this->value === 2) {
            return true;
        }

        if ($this->isEven()) {
            return false;
        }

        for ($i = 3; $i <= ceil(sqrt($this->value)); $i = $i + 2) {
            if ($this->value % $i == 0) {
                return false;
            }
        }

        return true;
    }