Install PHP 5.3 + Apache on ubuntu 9.10 using dotdeb repositories, via apt-get

PHP 5.3 isn’t proposed by default within the default ubuntu 9.10 repositories.
Here are the steps needed to quickly install Apache + PHP 5.3 in an ubuntu linux environment.
All in two command lines
Just copy the two line above to get php 5.3 working
sudo su
cd /tmp && mkdir php53 && cd php53 && [...]