Do I need to specify a file as well as a path for session data?
Just a path should be fine.
But, if you're on a hosting service be aware that you might not always be on the same server. You might be on a server farm where /tmp is a different local hard disk on each request.
So you might have to try a directory that's 'persistent', in a filesystem that's shared across servers. There might be a special place for this (like on SourceForge's servers now) or you might have to take over a locked directory in your own web space. Check with the documentation or support for your hosting service if you have to do something like that.
Thanks for helping Brion!
Tech support at the hoisting service indicates this is running as php-cgi, so it should be able to write to a path anywhere under my home directory. They also indicated I need to use a custom php.ini file instead of using an .htaccess file. I've now tried that--setting the path in that to my home directory, but the session data is still being lost.
Is there some problem associated with running in php-cgi and/or using this custom php.ini file?
Thanks, Scott