如何在使用会话登录后更改导航栏，在登录之前显示该页面

我尝试并想要(index.php)我的主页，它显示网页，但导航栏极右显示登录菜单。登录后，将创建会话并将导航栏从登录更改为注销。然后它的会话也访问剩余的页面。

login。

<?php ob_start(); ?>
<?php
 session_start(); 
 require_once 'dbconnect.php';
 if (isset($_SESSION['userSession'])!="") {
       header("Location: index.php");
      exit;
  }
 if (isset($_POST['btn-login'])) {
 $email = strip_tags($_POST['email']);
 $password = strip_tags($_POST['password']);
 $email = $DBcon->real_escape_string($email);
 $password = $DBcon->real_escape_string($password);
 $query = $DBcon->query("SELECT user_id, email, password FROM tbl_users                   WHERE email='$email' and password='$password'");
    $row=$query->fetch_array();
 $count = $query->num_rows; // if email/password are correct returns must be 1 row
if ($count==1) {
    $_SESSION['userSession'] = $row['user_id'];
header("Location: index.php");
} else {
    $msg = "<div class='alert alert-danger'>
                <span class='glyphicon glyphicon-info-sign'></span> &nbsp; Invalid Username or Password !
            </div>";
}
$DBcon->close();
}
?>

index . php

<?php 
 session_start();
 include_once 'dbconnect.php';
 if (!isset($_SESSION['userSession'])) {
    header("Location: login.php");
  }
 $query = $DBcon->query("SELECT * FROM tbl_users WHERE user_id=".$_SESSION['userSession']);
 $userRow=$query->fetch_array();
 $DBcon->close();
 ?>

导航条

<?php   
if(isset($_SESSION['userSession'])) {
    echo '<li class="dropdown"><a href="logout.php"><span>Log Out</span></a>   </li>';
} else {
    echo '<li class="dropdown"><a href="login.php"><span>Log In</span></a></li>';
} 
?>

这是我的编码。登录前无法打开index.php文件。现在我需要你的帮助，index.php文件打开没有登录，但导航栏显示登录菜单。一旦登录，它将取代注销菜单。

谢谢你提前回复我