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sudo apt-get remove phpunit
sudo pear channel-discover pear.phpunit.de
sudo pear channel-discover pear.symfony-project.com
sudo pear channel-discover components.ez.no
sudo pear clear-cache
sudo pear update-channels
sudo pear upgrade-all
sudo pear install --alldeps phpunit/PHPUnit
sudo pear install --force --alldeps phpunit/PHPUnit
sudo apt-get install php5-curl
sudo pear install --alldeps phpunit/PHPUnit_Selenium
sudo pear install phpunit/PHP_CodeCoverage
sudo pear clear-cache
sudo pear uninstall phpunit/PHPUnit
sudo pear uninstall phpunit/PHPUnit_Selenium
sudo pear uninstall phpunit/PHPUnit
sudo pear install phpunit/PHPUnit
sudo pear install phpunit/PHPUnit_Selenium
phpunit --version
sudo pear uninstall phpunit/PHPUnit_Selenium
sudo pear install phpunit/PHPUnit
sudo pear uninstall phpunit/PHPUnit
wget https://phar.phpunit.de/phpunit.phar
chmod +x phpunit.phar
mv phpunit.phar /usr/local/bin/phpunit
sudo mv phpunit.phar /usr/local/bin/phpunit
exit
phpunit --version
phpunit --version
phpunit --version
phpunit --version

Sources: first, second, third.