Depending on what you are using sessions or cookies you should create function similar to this
function checkLogged() {
if (!isset($SESSION['user'])) {
if (mysql_num_rows(mysql_query("SELECT * FROM users WHERE userid = '".@$SESSION['user']['userid']."'")) != 1) {
return false;
} else {
return true;
}
} else {
return false;
}
}
[/code]
Basically true means there logged in
Than on all files requiring it do this
if (checkLogged() != true) {
//this is a guest show login form
} else {
//user logged in
}