Session - funny or magic?

Generally I don't believe in magic, but... I tried to use 'session' machanism. So I typed "session_start()" and so on, configured path to store session info. Surely before sending any headers. I defined variable "level", accesed by $_SESSION["level"]. Sesion files were being created in correct directory, but files had zero length and mentioned variable was not changing... was not even set. Register_globals was ON, everything should work. But didn't. Turning off register_globals didn't change anything. I have added "counter" - just example from php manual. Initially nothing happened until I changed acces mode to counter from $_SESSION["counter"] to $HTTP_SESSION_VAR["counter"]. "Counter" was still unaffected and unaccesible, but "level" (still accesed by $_SESSION["level"]) was! I'd love to know WHY. I'm afraid it can stop working "without" reason, just as started. Could anybody explain me what's going on?