src/web/Request.php
protected function loadCsrfToken()
    {
        if ($this->enableCsrfCookie) {
            return $this->getCookies()->getValue($this->csrfParam);
        } else {
            return Yii::$app->getSession()->get($this->csrfParam);
        }
    }
protected function loadCsrfToken()
    {
        if ($this->enableCsrfCookie) {
            return $this->getCookies()->getValue($this->csrfParam);
        } else {
            return Yii::$app->getSession()->get($this->csrfParam);
        }
    }
Loads the CSRF token from cookie or session.
@return string the CSRF token loaded from cookie or session. Null is returned if the cookie or session
does not have CSRF token.