How do I install different (upgrade or downgrade) PHP version in still supported Ubuntu release?

I am trying to update to install PHP 5.5, 5.6 or 7.0 in Ubuntu 12.04/14.04/16.04 LTS, and I can only get PHP 5.3.10 for Ubuntu 12.04, PHP 5.5.9 for Ubuntu 14.04 or PHP 7.0.4 for Ubuntu 16.04 using official repositories (using apt-get).

I'm not really sure how to do a manual update – as I need:

• to play around with the new(est) PHP features
• to install older PHP version due support in the software

You could use a PPA to stay up-to-date with PHP or install previous version of PHP (f.e. PHP 5.6 on Ubuntu 16.04 LTS).

The most widely used repositories come from Ondřej Surý, the Debian PHP maintainer:

• ppa:ondrej/php (for PHP 5.6/7.0/7.1 co-installable versions, PHP 5.5 is no longer supported)

The php5 compat packages:

• ppa:ondrej/php5-compat (for dummy php5 to satisfy dependencies in the old packages)

(Click here for instructions on using PPAs.)

PHP PPAs previously contained Apache 2.4 update. This is no longer a case, you need to add separate Apache 2.4 repository:

• ppa:ondrej/apache2 (for Apache 2.4)

If you want use these PPAs, do this:

ppa:ondrej/php (for PHP 5.6/7.0/7.1)

sudo add-apt-repository ppa:ondrej/php
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install php7.1   # for PHP 7.1
sudo apt-get install php7.0   # for PHP 7.0
sudo apt-get install php5.6   # for PHP 5.6

To switch between installed versions use

sudo update-alternatives --config php

Then you must set Apache to work with right version:

sudo a2dismod php7.1         # unload the current version
sudo a2enmod  php5.6         # load the version you need
sudo service apache2 restart # restart webserver to apply

ppa:ondrej/php5-compat (for php5 compat)

sudo add-apt-repository ppa:ondrej/php
sudo add-apt-repository ppa:ondrej/php5-compat
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install php5 # this will pull php5.6 package 

If you don't have add-apt-repository binary do the following:

sudo apt-get install python-software-properties

Precautions:

• Are PPA's safe to add to my system and what are some "red flags" to watch out for?
• Each version of PHP has its own set of modules and php.ini.

PHP 5.4

Prepackaged latest PHP 5.4 now resides in separate PPA: ppa:ondrej/php5-oldstable

Please be aware that PHP 5.4 has reached its end of life on 3. September 2015 and it doesn't receive any security updates. It's recommended to migrate to at least PHP 5.6 that will receive security updated till 31. December 2018.

It will take some time for PHP 5.4 packages to make their way into Ubuntu, as there are an awful lot of dependency checks to do. Ubuntu 12.04 is now in beta so I doubt they'll upgrade it to PHP 5.4 (although I have no official or inside knowledge of this) -- it probably won't appear until Ubuntu 12.10 in October this year. If you want it sooner than that, you'll either have to find a third-party package or learn to build it yourself from the source code.