Файл:  /etc/apt/sources.list:

deb http://ftp.de.debian.org/debian/ squeeze main

премахваме php 5
apt-get remove --purge `dpkg -l | grep php | grep -w 5.4 | awk '{print $2}' | xargs`

Виждаме какви версии можем да инсталираме:

apt-cache showpkg php5

Provides:
5.4.4-14 —
5.3.3-7+squeeze14

Инсталираме пакетите ;)
apt-get install php5=5.3.3-7+squeeze14 php5-cli=5.3.3-7+squeeze14 php5-common=5.3.3-7+squeeze14 php5-mysql=5.3.3-7+squeeze14 libapache2-mod-php5=5.3.3-7+squeeze14