Upgrading PHP4 to PHP5 on a vanilla Debian Etch / Plesk 8.6.0 system with apt-get eventually renders Plesk unusable. I had to learn this the hard way. DO NOT UPGRADE TO PHP5 BEFORE DOING THE FOLLOWING STEPS:
BEFORE upgrading anything with apt-get, add the following apt source to /etc/sources.list:
deb http://autoinstall.plesk.com/debian/PSA_8.6.0 etch all
save it and do
Now you may upgrade the system with
If everything works fine, now you HAVE TO install the PHP5 support package for Plesk by hand:
apt-get install psa-php5-configurator
As a final step, restart Plesk with
…and log into Plesk.
Tadaaa! Plesk runs with full PHP5 support now.
solution found at this site