This how-to allowed me to have xampp and cacti working, at last, in Ubuntu Dapper, very clear instructions. Thanks.

I found a typo in crontab. Instead of this line:

/5 * * * * deadwait /opt/lampp/bin/php /opt/lampp/htdocs/cacti/poller.php > /dev/null 2>&1

you may want to use this (I added the asterisk):

*/5 * * * * deadwait /opt/lampp/bin/php /opt/lampp/htdocs/cacti/poller.php > /dev/null 2>&1

Even better if you use www-data instead of deadwait. www-data is already a user in Ubuntu.

 */5 * * * * www-data /opt/lampp/bin/php /opt/lampp/htdocs/cacti/poller.php > /dev/null 2>&1

 Now, in order to enable cacti to write, we should change some rights on a couple of folders:

sudo chown -R www-data:www-data /opt/lampp/htdocs/cacti/log

and:

sudo chown -R www-data:www-data /opt/lampp/htdocs/cacti/rra