session_id() creates new session every reload

Question

I've read this before:

How to fix “Headers already sent” error in PHP

I have a session page, when I refresh/reload it, it creates a new session id!

All my codes :

<?php   
    ob_start();
    session_start();

    if(empty($_SESSION['test'])){
        $_SESSION['test'] = session_id();   
    }   
    echo $_SESSION['test'];
?>

but when I refresh my page, it creates a new session id ! why?