To upgrade to a newer version of PHP 5.6 on Ubuntu 14.04, you can use Ondřej Surý’s PPA. He is one of the Debian maintainers of the php5 package. To add the PPA and upgrade your packages, run:
sudo apt-get install software-properties-common
sudo add-apt-repository ppa:ondrej/php5-5.6
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install php5
Though remember to be careful when doing an upgrade of PHP outside of the normal distro repositories, some of the additional modules that you might rely on could be missing for that version. I generally recommend that you shouldn’t do such a major upgrade unless you specifically need a new feature from the newer release. Remember that Ubuntu backports security patches to their “Long Term Support” releases. So if your concern is making sure you have the most recent security fixes, simply updating your packages as normal should provide them.