Taken from https://bugs.launchpad.net/ubuntu/+source/phpunit/+bug/701544 with reference from http://stackoverflow.com/questions/1528717/phpunit-require-once-error

Install PHPUnit on Ubuntu 12.04

sudo apt-get remove phpunit
sudo apt-get upgrade pear
sudo pear channel-discover pear.phpunit.de
sudo pear channel-discover pear.symfony.com
sudo pear channel-discover components.ez.no
sudo pear update-channels
sudo pear upgrade-all
sudo pear install --alldeps phpunit/PHPUnit