"Have you registered the channel required to install YAML?
pear channel-discover pear.symfony-project.com
Depending on how pear is installed and configured, you may need to preface the command with ""sudo"" to run it as root.
From the PHPUnit manual:
The PEAR channel (pear.phpunit.de) that is used to distribute PHPUnit needs to be registered with the local PEAR environment. Furthermore, a component that PHPUnit depends upon is hosted on the Symfony Components PEAR channel (pear.symfony-project.com)."