I started trying to test out
JFFNMS on Debian months ago and eventually gave up when I couldn't get the poller to work. It turns out the poller takes advantage of
PHP-CLI and that PHP-CLI (and PHP-CGI) has a seperate php.ini file where you need to configure extensions/modules/whatever you decide to call them. So if you're having the same problem open up /etc/php4/cli/php.ini in a text editor and un-comment "extension=mysql.so" (NOTE: If you're not using MySQL you're going to need to change mysql.so to whatever extension is appropriate for the database you are using).
Now, to test the change you just made, drop back to the command line and run "php /usr/share/jffnms/engine/poller.php <host id>" where <host id> is the host id corresponding to a valid host in JFFNMS DB and you should see a lot of relevant output. If not I would suggest starting your troubleshooting adventure
here.