I want to downgrade PHP for testing purpose. How i can safely do it? I have
Distributor ID: Ubuntu Description: Ubuntu 14.04.1 LTS Release: 14.04 Codename: trusty
you can install both php versions parallel e.g. PHP 5.5x and 5.3x or even three PHP versions parallel e.g. PHP 5.5x , 5.4x and 5.3x using fastcgi.
Just follow these instructions:
The tutorial is based on Ubuntu 14.