how to restrict users from entering admin pages from url

[promise4]

The following is my block of code and still from the url user can access the admin page without been logged in.
Please, i need help

if(!isset($_SESSION["useradmin"]))
{
  header('Location: login_admin.php?error=Access denied'); 
  exit();
}

[Dormilich]

The following is my block of code and still from the url user can access the admin page without been logged in.

then obviously $_SESSION["useradmin"] is always set albeight with different values.