First, install PEAR.
sudo apt-get install php-pear
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
Then install PHPUnit. (the -a makes sure all dependency packages are also installed)
sudo pear install -a phpunit/PHPUnit
According to the latest PHPUnit installation documentation, you can install PHPUnit with the following commands (make sure you have updated PEAR first):
sudo pear config-set auto_discover 1
sudo pear install pear.phpunit.de/PHPUnit