PHP in Apache with the CMS Joomla(prior to 1.5 i.e. 1.0.x) wants to run with user apache and group apache:
So to get a web running with joomla I have found I need to chown it and chmod it thus-
[fedora core 5 paths]
expand the joomla files or upload them to /var/www/webxx which you have created in ispconfig with the user webxx_user and a suitable db and db user (webxx_db1 & webxx_u1 with 'somepassword')
This will give the files in /var/www/webxx/web the user and group ownership of webxx_user and webxx.
Then from /var/www/webxx run the following
chown -R -v -f apache:apache web/*
then run from the same place
chmod -R -v 775 web/*
once you have done this then run your joomla install from your web browser via your chosen dns. (not pretty but it works for me - you could probably write a shell script if you were doing it often.)
PHP5 in this fedora pefect install causes some issues and I haven't worked them out yet.