when does session file get written?

i am having problems setting a custom sessions directory using session_save_path(). i need to be able to load a $sessions_directory variable from a config file, and then use that as the argument to session_save_path(). but the problem is, it doesn't seem to be working: php is still saving to /tmp. at what point does the session get written? my code is roughly like session_start(); # get $session_directory and call session_save_path() with it session_register( $variables ); but the file is still written to /tmp. of course i cannot do the config part first and then call session_start() after that, because of the famous "Cannot send session cache limiter" headers issue. other than that, however, doing it in that order works. does anybody have any suggestions? why is the session written to /tmp even when i have told it to save to a different path? it appears the session is written right at session_start() and i am calling session_save_path() too late? thanks for any help, ryan php 4.0.1pl2 solaris 2.7 ./configure --with-mysql --with-apxs=/usr/local/apache/bin/apxs --with-dom