session does not work with mod-rewrited urls?

I have this page www.exmaple.com/username which is rewritten url, but the sessions does not work there.

That is, when I echo, it does not show the value, but all the other pages that have not been re-written are wo开发者_如何学JAVArking!

<?php session_start();
echo $_SESSION['user_id'];

?>

Is there something that I'm missing?

---

Does this rewrite involve a change of domain or subdomain in the URL? Cookies are, by default, accessible only to the same subdomain they were set on... and session identifiers are maintained by cookies.

---

Sessions are not affected by mod_rewrite, but you need to prepend every page with session_start().

---

You must call session_start() to read the session or $_SESSION will not be populated.