SESSION is not loading variables $ _SESSION ['']

I am making a SESSIONS-based user authentication system. I created a function that does the LOGOUT which looks like this:

session_destroy(); // Cancela/Exclui a sessão iniciada

unset($_SESSION['usuario']);

?>

<script type="text/javascript">

location.href='index.php';

</script>   

But by clicking the browser's "back" button, the script is not pointing to the home page. I put the following code on the protected page:

session_start();

if(!isset($_SESSION['usuario'])){
header("Location: index.php?error=2");
}

On the authentication page, my code looks like this:

$usuario= $row["usuario_id"];
$nivel = $row["nivel_usuario"];
$nome   = $row["nome"];
$fnc    = $row["funcao"];

    session_start();

    if($nivel == 2 ){


        $_SESSION['adm'] = "$nome";
        $_SESSION['usuario'] = "$usuario";

    } else {

        $_SESSION['nor'] = "$nome";
        $_SESSION['usuario'] = "$usuario";              

    }    

    echo '<script type="text/javascript">window.location = "pages/index.php?fnc=' .  $fnc  . '"</script>';

}

} else {

    echo '<script type="text/javascript">window.location = "index.php?erro=2"</script>';

}
}