Security patches for PHP 5.5 have ended! Get thee to 5.6
This is the list of currently supported PHP versions: http://php.net/supported-versions.php
While Vanilla 2.2 will support PHP 5.3, we strongly recommend upgrading to the newest version possible.
Even PHP 5.5 will be outside its security fixes as of 10 Jul 2016. That's 8 months from now. Vanilla 2.3 will absolutely require 5.4, and it will arrive soon than you expect. And it's possible we'll move to 5.5 minimum by the end of 2016.
So the short version is: You should be running PHP 5.6. It's important.