public function createNewToken($captcha = null, $answer = null, $expire = 300)
    {
        if ((null === $captcha) || (null === $answer)) {
            $captcha = $this->generateEquation();
            $answer  = $this->evaluateEquation($captcha);
        }
        $this->token = [
            'captcha' => $captcha,
            'answer'  => $answer,
            'expire'  => (int)$expire,
            'start'   => time()
        ];
        $_SESSION['pop_captcha'] = serialize($this->token);
        return $this;
    }