yii\captcha\CaptchaAction::run PHP Method

    public function run()
    {
        if (Yii::$app->request->getQueryParam(self::REFRESH_GET_VAR) !== null) {
            // AJAX request for regenerating code
            $code = $this->getVerifyCode(true);
            Yii::$app->response->format = Response::FORMAT_JSON;
            return ['hash1' => $this->generateValidationHash($code), 'hash2' => $this->generateValidationHash(strtolower($code)), 'url' => Url::to([$this->id, 'v' => uniqid()])];
        } else {
            $this->setHttpHeaders();
            Yii::$app->response->format = Response::FORMAT_RAW;
            return $this->renderImage($this->getVerifyCode());
        }
    }