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

apt-get update

Now you may upgrade the system with

apt-get upgrade

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

/etc/init.d/psa restart

…and log into Plesk.

Tadaaa! Plesk runs with full PHP5 support now.

solution found at this site

(May 2009)

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