Source: https://www.tecmint.com/install-different-php-versions-in-ubuntu/

INSTALL:

$ sudo apt install php

$ sudo apt install python-software-properties
$ sudo add-apt-repository ppa:ondrej/php

$ sudo apt-get update

$ sudo apt install php5.6   [PHP 5.6]
$ sudo apt install php7.0   [PHP 7.0]
$ sudo apt install php7.1   [PHP 7.1]

------------ Install PHP Modules ------------
$ sudo apt install php5.6-cli php5.6-xml php5.6-mysql 
$ sudo apt install php7.0-cli php7.0-xml php7.0-mysql 
$ sudo apt install php7.1-cli php7.1-xml php7.1-mysql

VERISON CHANGE:

------------ Set Default PHP Version 5.6 ------------
$ sudo update-alternatives --set php /usr/bin/php5.6

$ sudo a2dismod php7.0
$ sudo a2enmod php7.1
$ sudo systemctl restart apache2

------------ For PHP 5.6 ------------
$ sudo update-alternatives --set php /usr/bin/php5.6
$ php -i | grep "Loaded Configuration File"
------------ For PHP 7.0 ------------
$ sudo update-alternatives --set php /usr/bin/php7.0
$ php -i | grep "Loaded Configuration File"
------------ For PHP 7,1 ------------
$ sudo update-alternatives --set php /usr/bin/php7.1
$ php -i | grep "Loaded Configuration File"