Session files in /tmp are zero length

Hi.. I'm migrating a web application from PHP 4.0.3 on Linux to 4.2.2 and having trouble with the session variables. The code has various statements like: session_register("varname"); I get the sess_... files in /tmp, but they're all zero length, so of course the application doesn't work... register_globals is ON and I've even tried sticking a

    global $varname;
ahead of the session_register, but nothing seems to force it to save it in the sess_... file.

Any suggestions for this relative novice? Lee Stewart