session_is_registered gets session values only after doing some output first !?

I try to check if a session is registered. This works fine if I first do some output echo "abc"; if (!session_is_registered('userid')) { do_something(); } else { redirect_to_somewhere(); } and I can't redirect to another page ... If I comment out the echo statement I get always false and $_SESSION is empty, but I could redirect somewhere... I tried php 4.3.1 and 4.3.7 with apache 2.0.45 both the same. On a Windows box the same program runs fine. Can anybody help? _________________________________________________________________________ Frank Rust, Technische Universität, Institut für Theoretische Informatik Tel.: +49 531 391 9525 Postfach 3329, D-38023 Braunschweig Fax.: +49 531 391 9529 Mühlenpfordtstr. 22-23, D-38106 Braunschweig