In production, we use Ubuntu 9.10, because of the need of the newer kernel. Now we also need 5.3 PHP, because of Symfony, so I started searching.
I’ve found a post on JMOZ blog, about installing 5.3 PHP from dotdeb packages, but when I tried, I recieved some more errors over the one libicu38 mentioned in the post, so I decided to write them down.

The needed additional apt sources:

# for PHP 5.3
deb http://php53.dotdeb.org stable all
deb-src http://php53.dotdeb.org stable all

# for libicu38
deb http://security.ubuntu.com/ubuntu jaunty-security main
deb-src http://security.ubuntu.com/ubuntu jaunty-security main

# for libltdl3
deb http://archive.ubuntu.com/ubuntu hardy main
deb-src http://archive.ubuntu.com/ubuntu hardy main
deb http://archive.ubuntu.com/ubuntu hardy-updates main
deb-src http://archive.ubuntu.com/ubuntu hardy-updates main