Иногда нужно и такое когда юзаете какие то старые движки.
Добавить репозиторий из PPA:
add-apt-repository ppa:txwikinger/php5.2
либо самостоятельно прописать в /etc/apt/sources.list
Внимание: если у вас Ubuntu 10.10 (а не 10.04), то вместо lucid пишите maverick
Добавить PGP-ключ
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 9CC59506
Запретить обновлять PHP (правим файл /etc/apt/preferences.d/php)
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
Запросим пакейджи
apt-get update
Ну и наконец произведем установку PHP 5.2
apt-get dist-upgrade