It's really such a headache when I migrated to Ubuntu 14.04. Lots of headaches! But anyway, here we go:
First you need to install pear first by doing the following command on Terminal:
sudo apt-get install php-pear
Then add pear channel:
sudo pear channel-discover pear.phing.info
sudo pear install -Z phing/phing
The -Z option will download .tar file instead of the damn .tgz (which always throws this error):
could not extract the package.xml file from "/build/buildd/php5-5.5.9+dfsg/pear-build-download/phing-2.9.1.tgz"
Download of "phing/phing" succeeded, but it is not a valid package archive
Error: cannot download "phing/phing"
Hopefully it works on your end. Please comment below if it worked.