inject vars to session

varstosession

protected $_paramsToTrack = array("offer_followup");

    /**
     * Capture any listened for parameters, and inject them into the session
     * under a variable called : ProductOfferUrlParams
     *
     * You must clear this variable with a call to
     *
     * Mage::getSingleton('customer/session')->unsProductOfferUrlParams();
     *
     * if the usage is no longer required, and it may trigger actions no longer
     * required.
     *
     *
     * @param Varien_Event_Observer $observer
     *
     * @return Viyet_ProductOffers_Model_Observer
     */
    public function controller_front_send_response_before(Varien_Event_Observer $observer)
    {
        try {
            if (Mage::getStoreConfig('productoffers/followup/disabled') == false) {
                $request = $observer->getFront()->getAction()->getRequest();
                $params = $request->getParams();
                if (0 === count(array_diff($this->_paramsToTrack, array_keys($params)))) {
                    Mage::getSingleton('customer/session')->setProductOfferUrlParams($params);
                }
            }
        } catch (Exception $e) {
            // log any issues, but allow system to continue.
            mage::logException($e);
        }

        return $this;
    }