我想在购物车内添加产品

In the below code I am able to create session and add first item in array. But I am not able to append the array in it. I am new to codigniter

   function guestcart($pID){

        $session = $this->session->userdata('guest');
        if(!empty($session)){  
            $addprod = $data;
            $addprod= array($pID);
            array_push($addprod,$data);

            echo 'Session Exist';
        } 
        else {     

    //set the session data    
        $data = array("guest",$pID);

    $session = $this->session->set_userdata('guest',$data);

    echo 'Session created';
}

}