Downgrading from PHP 5.3 to 5.2 on Debian Squeeze

Problem

You've installed PHP 5.3 on your Debian Squeeze system. Problem is, some web apps have trouble running on that version. You need to downgrade your system from 5.3 to 5.2.

Solution

Remove the PHP 5.3 packages from your system:

sudo aptitude purge `dpkg -l | grep php| awk '{print $2}' |tr "\n" " "`

Clean the cache just to be sure:

rm -f /var/cache/apt/archives/php5*

Use Karmiс for PHP packages:

echo -e "Package: php5\nPin: release v=karmic\nPin-Priority: 991\n" | sudo tee /etc/apt/preferences.d/php > /dev/null
apt-cache search php5-|grep php5-|awk '{print "Package:", $1,"\nPin: release v=karmic\nPin-Priority: 991\n"}'|sudo tee /etc/apt/preferences.d/php > /dev/null

Add Ubuntu Karmic to source list:

cd /etc/apt/sources.list.d
sudo wget -O karmic.list "http://pastebin.com/download.php?i=q9ya307g"

(Update, October 18, 2012: The sources list has been updated, because Ubuntu no longer supports Karmic. Please leave a comment if this still doesn't work.)

Update the package database:

sudo apt-get update

If the command above produces this error:

W: GPG error: http://security.ubuntu.com karmic-security Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5
W: GPG error: http://archive.ubuntu.com karmic Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5
W: GPG error: http://archive.ubuntu.com karmic-updates Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5

Then import the required keys and add them to your list of trusted keys:

gpg --keyserver hkp://subkeys.pgp.net --recv-keys 40976EAF437D05B5
gpg --export --armor 437D05B5 | sudo apt-key add -

gpg --keyserver hkp://subkeys.pgp.net --recv-keys 40976EAF437D05B5
gpg --export --armor 40976EAF437D05B5 | sudo apt-key add -

gpg --keyserver hkp://subkeys.pgp.net --recv-keys 40976EAF437D05B5
gpg --export --armor 40976EAF437D05B5 | sudo apt-key add -

Finally, install PHP 5.2:

sudo apt-get install -t karmic php5-cli php5-cgi libapache2-mod-php5

UPDATE: Using the Ubuntu sources to download old packages works on Debian. So far, it works on my system, but you may need to use the Debian sources just in case.