For an older PHP project, I needed to install an older version of PHP. This is what I did to set that up.

Installing a different PHP version

sudo add-apt-repository ppa:ondrej/php
sudo apt-get update
sudo apt-get install -y php7.1

Rebinding php to required version

Some of these binds are probably not need. I think the main ones, at least for my use case, were php and phar.

sudo update-alternatives --set php /usr/bin/php7.1
sudo update-alternatives --set phar /usr/bin/phar7.1
sudo update-alternatives --set phar.phar /usr/bin/phar.phar7.1
sudo update-alternatives --set phpize /usr/bin/phpize7.1
sudo update-alternatives --set php-config /usr/bin/php-config7.1

For some reason the --set flag stopped working, so I had to use:

sudo update-alternatives --config php
sudo update-alternatives --config phar

etc. And update each one with the terminal prompt options for each.

p.s. If using PHP-FPM, you could also set up different server conf files and point the FPM path to the version you need. My need was just because I was using the command line in the older project.