$getRegistered = $conn->prepare("SELECT id, user FROM register_results WHERE event_id = :event_id");
            $getRegistered->bindParam(":event_id", $_SESSION['event_id']);
            $getRegistered->execute();

            $list = array();
            $headers = array();
            $users = array();

            while($result = $getRegistered->fetch(PDO::FETCH_ASSOC)){
                // make new user object
                $user_object = array();
                $user_object['id'] = $result['id'];
                $user_object['user'] = json_decode($result['user']);
                //get headers by looping each result['user']
                foreach (json_decode($result['user']) as $key => $value) {
                    //if new value push
                    if(!in_array($key, $headers)){
                        array_push($headers, $key);
                    }
                }
                $users[] = $user_object;
            }

            //add missing keys and values
            foreach($users as $user){
                //get user from user row
                $row = $user['user'];
                //loop headers for each row
                foreach($headers as $header){
                    //check if $header found in each row
                    if (!array_key_exists($header,$row)){
                        //if not found
                        $row->$header = "-";
                    }
                }



            }


            $list['headers'] = $headers;
            $list['users'] = $users;

            echo json_encode($list);