Canonical has removed the repositories for karmic release so the old method to downgrade PHP stopped working, here is another way to downgrade PHP to 5.2.x
First add the PPA to your repository list
$ sudo add-apt-repository ppa:txwikinger/php5.2
if you are in Lucid then that's it, if not edit the recently added entry and change your distribution codename for lucid
$ sudo vi /etc/apt/sources.list.d/txwikinger-php5_2-oneiric.list
for example I have
deb http://ppa.launchpad.net/txwikinger/php5.2/ubuntu oneiric main
deb-src http://ppa.launchpad.net/txwikinger/php5.2/ubuntu oneiric main
and I edited to
deb http://ppa.launchpad.net/txwikinger/php5.2/ubuntu lucid main
deb-src http://ppa.launchpad.net/txwikinger/php5.2/ubuntu lucid main
Now you need to add the pinpoint for php packages
$ sudo vi /etc/apt/preferences.d/php
And put the content
Package: libapache2-mod-php5
Pin: version 5.2.10*
Pin-Priority: 991
Package: libapache2-mod-php5filter
Pin: version 5.2.10*
Pin-Priority: 991
Package: php-pear
Pin: version 5.2.10*
Pin-Priority: 991
Package: php5
Pin: version 5.2.10*
Pin-Priority: 991
Package: php5-cgi
Pin: version 5.2.10*
Pin-Priority: 991
Package: php5-cli
Pin: version 5.2.10*
Pin-Priority: 991
Package: php5-common
Pin: version 5.2.10*
Pin-Priority: 991
Package: php5-curl
Pin: version 5.2.10*
Pin-Priority: 991
Package: php5-dbg
Pin: version 5.2.10*
Pin-Priority: 991
Package: php5-dev
Pin: version 5.2.10*
Pin-Priority: 991
Package: php5-gd
Pin: version 5.2.10*
Pin-Priority: 991
Package: php5-gmp
Pin: version 5.2.10*
Pin-Priority: 991
Package: php5-ldap
Pin: version 5.2.10*
Pin-Priority: 991
Package: php5-mhash
Pin: version 5.2.10*
Pin-Priority: 991
Package: php5-mysql
Pin: version 5.2.10*
Pin-Priority: 991
Package: php5-odbc
Pin: version 5.2.10*
Pin-Priority: 991
Package: php5-pgsql
Pin: version 5.2.10*
Pin-Priority: 991
Package: php5-pspell
Pin: version 5.2.10*
Pin-Priority: 991
Package: php5-recode
Pin: version 5.2.10*
Pin-Priority: 991
Package: php5-snmp
Pin: version 5.2.10*
Pin-Priority: 991
Package: php5-sqlite
Pin: version 5.2.10*
Pin-Priority: 991
Package: php5-sybase
Pin: version 5.2.10*
Pin-Priority: 991
Package: php5-tidy
Pin: version 5.2.10*
Pin-Priority: 991
Package: php5-xmlrpc
Pin: version 5.2.10*
Pin-Priority: 991
Package: php5-xsl
Pin: version 5.2.10*
Pin-Priority: 991
Package: php5-mcrypt
Pin: version 5.2.6*
Pin-Priority: 991
Package: php5-imap
Pin: version 5.2.6*
Pin-Priority: 991
Now it's time to update the pachage list
$ sudo apt-get update
Everything should run without errors or warnings, now install PHP 5.2
sudo apt-get install php-pear php5-dev php5-gd php5-mysql php5-sqlite libapache2-mod-php5