WEll as you may know the Session start must be before any HTML at all...at the very top of the page.
Also
After the user has logged in... on the other pages you also have to put Session_Start. Then use the
If (session_is_registered("Sessname")) {
Do stuff
}
So on log in page have this>
$SessName=$user_name.date("dmY");
session_start("SessName");
session_register("SessName");
Then on your next pages have this
session_start();
If (session_is_registered("Sessname")) {
Do stuff
}