Ubuntu 10.04 LTS Lucid provides PHP 5.3.2, which is missing some key goodies and has some well known bugs. At the time of writing ubuntu have not backported a newer version to Lucid, however there is an outstanding bug request for it.
In the mean time ive noticed some people are manually compling a new version for their LTS, but thanks to a developer called Fabián Arias there is no longer a need to do this. He has created a PPA for other ubuntu users which will allow you to easily update your PHP (to 5.3.5 at the time of writing).
Id suggest heavy testing before you use this in a live environment, but for those willing to give it a go, just use the following steps;
sudo apt-get install python-software-properties sudo add-apt-repository ppa:fabianarias/php5 sudo apt-get update sudo apt-get upgradeWarning, im not sure if this PPA is being updated with the latest php security patches. Use at your own risk!