(from http://www.joyceleong.com/log/installing-phantomjs-on-ubuntu/)
cd ~
wget http://phantomjs.googlecode.com/files/phantomjs-1.9.1-linux-x86_64.tar.bz2
sudo mv ~/phantomjs-1.9.1-linux-x86_64.tar.bz2 /usr/local/share
cd /usr/local/share
sudo tar xvf phantomjs-1.9.1-linux-x86_64.tar.bz2
sudo ln -s /usr/local/share/phantomjs-1.9.1-linux-x86_64 /usr/local/share/phantomjs
sudo ln -s /usr/local/share/phantomjs/bin/phantomjs /usr/local/bin/phantomjs
phantomjs --version