- find out packages which came from php5.3:
dpkg -l | grep php | grep “5.3″ | awk ‘{print $2}’

- Use apt pinning: Add those packages to /etc/apt/preferences, like
Package: php5
Pin: release a=oldstable
Pin-Priority: 996

- Add as the last entry in /etc/apt/preferences:
Package: *
Pin: release o=Debian
Pin-Priority: -1

-Use apt-cache policy php5 before installing to see if your pinning was successful

- Pretend to reinstall those packages found in the first step in simulation mode:
apt-get install –simulate -t lenny –reinstall

- Include all php5 related package the system wants to remove in /etc/apt/preferences the same way as described above

- Retry reinstallation simulation and repeat apt pinning until no more packages you really need, would be removed

- Do the reinstallation (suppress the –simulate switch)

- Find out which additional packages from lenny are needed: Watch the logs for error messages, stating a file was not found. Search the corresponding package and pin and install it (or scroll back in your terminal, if you still have the messages from the unlucky upgrade to php5.3)

Like this I could downgrade php to 5.2 and let my horde and roundcube installation run under squeeze.

Please correct, if this is incomplete or wrong.

#Script to install PHP 5.2 from 9.10 on 10.10
# And pin it so it does not get updated

PKGS=`dpkg -l | grep php | awk ‘{print $2}’`

apt-get remove $PKGS

sed s/maverick/karmic/g /etc/apt/sources.list |
tee /etc/apt/sources.list.d/karmic.list

mkdir -p /etc/apt/preferences.d/

for PACKAGE in $PKGS
do
echo “Package: $PACKAGE
Pin: release a=karmic
Pin-Priority: 991
” | tee -a /etc/apt/preferences.d/php
done

apt-get update

apt-get install $PKGS