Ubuntu 10.04 Specific : Downgrade to php 5.2 before install

What is explained here is specific to Ubuntu 10.04, it hasn't been tested on further versions and you should anyway use Ubuntu 10.04 (LTS) on production servers.

Follow this procedure :

sudo apt-get install python-software-properties

add-apt-repository ppa:txwikinger/php5.2

Create the file /etc/apt/preferences.d/php and input the following code in it :

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

Then proceed Aegir install as explained at http://community.aegirproject.org/installing/debian