When php is upgraded to php5 on Debian, the Plesk panel goes down.

Symptoms:
If you try to upgrade libapache2-mod-php4 to libapache-mod-php5 package, apt-get tries to remove package psa with all dependencies, in accordance to dependency listing of psa-php4-configurator:

# apt-get install libapache2-mod-php5 Reading Package Lists... Done
Building Dependency Tree... Done
Suggested packages:
php-pear
The following packages will be REMOVED:
libapache2-mod-php4 libpam-plesk psa psa-api psa-horde psa-hotfix3 psa-hotfix4 psa-hotfix5 psa-hotfix6 psa-imp psa-php4-configurator
The following NEW packages will be installed:
libapache2-mod-php5

Resolution:
To solve this issue you should install psa-php5-configurator as well:
# apt-get install psa-php5-configurator libapache2-mod-php5

This package is available in Plesk Debian repository in 'dist-*/contrib/'.

You can download it manually from http://autoinstall.plesk.com/debian/PSA or configure Debian repository:

deb http://autoinstall.plesk.com/debian/PSA_ all

for example for Plesk 8.2.1 and Debian 4:

deb http://autoinstall.plesk.com/debian/PSA_8.2.1 etch all