How to install older versions of PHP in Ubuntu

Sometimes, older versions of PHP are needed in local environments when dealing with legacy sites.

I'll show you how to install multiple versions of PHP in Ubuntu. This is specifically for a Magento local development environment but is relevant for any PHP application. 

Install additional versions of PHP

1. Add the ondrej/php repository:

sudo add-apt-repository ppa:ondrej/php

2. Install php7.x. Replace the "x" with the exact version you need. I will show php7.3 for demonstration purposes, but you can change it to the desired version:

sudo apt-get update
sudo apt-get install php7.3

3. Install Magento-required PHP modules:

sudo apt install php7.3-cli php7.3-fpm php7.3-json php7.3-pdo php7.3-mysql php7.3-zip php7.3-gd php7.3-mbstring
php7.3-curl php7.3-xml php7.3-bcmath php7.3-json php7.3-soap php7.3-intl php-libsodium

4. To change between PHP versions from the CLI (Note: you will need to configure apache or nginx to use the correct PHP version. This is for CLI only.)

sudo update-alternatives --config php

To make the PHP switch a little more convenient

1. Create a file named:

/usr/bin/changephp

2. Make the file executable:

sudo chmod 755 /usr/sbin/changephp

3. Save the contents below into the changephp file:

#!/bin/bash
sudo update-alternatives --config php

4. You can change the CLI php version by typing changephp from anywhere. It will bring up a dialog similar to this:

That's it! Now you will be able to change PHP on-the-fly to help support older PHP applications. The goal must be to get the sites on the latest versions of Magento and PHP as soon as possible though!