PHPUnit

First, install PEAR.

sudo apt-get install php-pear phpunit

Next, tell PEAR to update its own channel.

sudo pear channel-update pear.php.net

Then, tell PEAR to upgrade itself to the newest version.

sudo pear upgrade-all

You should now have the newest version of PEAR installed.

To install PHPUnit, let PEAR know where to find PHPUnit.

sudo pear channel-discover pear.phpunit.de
sudo pear channel-discover pear.symfony-project.com
sudo pear channel-discover pear.symfony.com

Then install PHPUnit. (the -a makes sure all dependency packages are also installed)

sudo pear install -a phpunit/PHPUnit

I think this is working.. =)