Close PHP Session

Hello

In my php file I have the code

  

page.php

session_start();
$valor = isset($_SESSION['usuario']) ? 'S' : 'N';
if($valor == 'N'){
    header('location: .');
}
echo "Sesssao: ".$valor;

To exit, in another file I do when clicking the link to exit that calls this function

function sair(){
    session_destroy();
    unset($_SESSION['usuario']);
    $_SESSION['usuario'] = null;
    header('location: .');
    session_commit();
}

But how much I access the direct page that verifies the session, without going through the login he said that there is always a session will be if it is by the word session_start ();