Update PHP to the latest version on Debian Lenny

It's easier than you think.

First you have to add the dotdeb packages to the /etc/apt/sources.list file

deb http://packages.dotdeb.org stable all
deb-src http://packages.dotdeb.org stable all

if you want the php 5.3 branch, add these lines instead

deb http://php53.dotdeb.org stable all
deb-src http://php53.dotdeb.org stable all

You have then to install the gpg keys :

gpg --keyserver keys.gnupg.net --recv-key 89DF5277
gpg -a --export 89DF5277 | sudo apt-key add -

(remove "sudo" if you're already root)

Then do :

apt-get update
apt-get upgrade

Here you have, the latest PHP version (5.2.14 or 5.3.3 at this moment)