Close PHP Session

Question

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 by clicking on the exit link that calls this function

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

But when I access the direct page that checks the session, without going through the login he said that there is always session is whether it has the word session_start();