framework/captcha/CaptchaValidator.php
public function createCaptchaAction()
    {
        $ca = Yii::$app->createController($this->captchaAction);
        if ($ca !== false) {
            /* @var $controller \yii\base\Controller */
            list($controller, $actionID) = $ca;
            $action = $controller->createAction($actionID);
            if ($action !== null) {
                return $action;
            }
        }
        throw new InvalidConfigException('Invalid CAPTCHA action ID: ' . $this->captchaAction);
    }
public function createCaptchaAction()
    {
        $ca = Yii::$app->createController($this->captchaAction);
        if ($ca !== false) {
            /* @var $controller \yii\base\Controller */
            list($controller, $actionID) = $ca;
            $action = $controller->createAction($actionID);
            if ($action !== null) {
                return $action;
            }
        }
        throw new InvalidConfigException('Invalid CAPTCHA action ID: ' . $this->captchaAction);
    }
Creates the CAPTCHA action object from the route specified by [[captchaAction]].
@return \yii\captcha\CaptchaAction the action object
@throws InvalidConfigException