php - session not working and return null

am trying to get the id in another page bu i always got nulls whats wrong with my code here ? and how to fix it ? ** the problem is in ->

public function login($email, $password) {
        $db = new database(); //connection with database     
        $q = "select * from user where
                email='$email'
                 AND 
                password='$password'";
        $db->pQuery($q);
        $userSet = $db->fetchAll();
       // var_dump($userSet);die;
        if (!empty($userSet)) {
          //  session_start();
            $_SESSION["username"] = $email;//not null
            $_SESSION["userid"] = $userSet['id'];//null
            print_r( $_SESSION["userid"]);
           var_dump($_SESSION["userid"]);die;
            header("location:search.php");
            exit();
        } else {
            return false;
        }
    } 

in the other page :

<?php
session_start();
$id  = $_SESSION['userid'];//==null
echo $id;//null here


var_dump($id);//null here