Running sites on different versions of PHP on the same server

This quick guide explains the steps needed to be able to run different versions of PHP on the same server (different virtual hosts or even different folders within the same site).

Prepare

If the ‘add-apt-repository’ command is missing, you need to install the package “software-properties-common” first:

apt install -y software-properties-common

Add the ondrej/php repository to your system:

add-apt-repository ppa:ondrej/php

apt update, upgrade:

apt update
apt upgrade

Install Apache fastCGI module:

apt install -y libapache2-mod-fcgid

For each version of PHP

As of 1 Jan 2021, PHP versions 7.4 (php7.4) and 8.0 (php8.0) are those with active support and updates. PHP version 7.3 (php7.3) will continue to receive critical updates until 1 Jan 2022. See Supported PHP Versions.

Install required packages

apt install -y php7.4 php7.4-fpm php7.4-mysql libapache2-mod-php7.4

Install other wanted modules for the same PHP version, for example these commonly used:

apt install -y php7.4-curl php7.4-gd php7.4-imagick php7.4-intl php7.4-mbstring php7.4-readline php7.4-json

Start the php-fpm service:

systemctl start php7.4-fpm

Check that the service is running:

systemctl status php7.4-fpm

For each version, also any custom configuration in php.ini has to be duplicated. In Ubuntu the php.ini files are located in the subfolders of /etc/php/x.x/ (one subfolder for each run environment, “apache2”, “cgi”, “cli”, “fpm”).

Set new default PHP version used by Apache

To set the default PHP version to use when not overridden by SetHandler below, use the commands a2dismod and a2enmod.
Set default to PHP 8.0

a2dismod php7.4
a2enmod php8.0
systemctl restart apache2

Set default to PHP 7.4

a2dismod php8.0
a2enmod php7.4
systemctl restart apache2

Configuring PHP version for virtual host or subfolder

Activate necessary Apache modules and restart Apache:

a2enmod actions fcgid alias proxy_fcgi
systemctl restart apache2

Use FilesMatch directive to set the PHP version:
FilesMatch is valid in both the virtualhost configuration and inside a Directory section.
To set PHP version globally for a virtual host, use it outside a Directory section.
The default PHP version can be set using ‘a2enmod php8.0’ (or any other version)

<FilesMatch \.php$>
  SetHandler "proxy:unix:/run/php/php7.4-fpm.sock|fcgi://localhost"
</FilesMatch>

Check the configuration for errors:

apachectl configtest

If result is “Syntax OK”, restart Apache:

systemctl restart apache2

Overriding PHP version using .htaccess

For this to work, “AllowOverride FileInfo” must be present for the directory (or above) in which the .htaccess file will be used to set the PHP version.
For the default virtual host, the DocumentRoot is set to /var/www/html, so to allow PHP version to be set by .htaccess at that level or below, the following must be present in the vhost configuration:

  <Directory /var/www/html>
    AllowOverride FileInfo
  </Directory>

When this has been set, FilesMatch and SetHandler (as described above) can be used within the .htaccess file. The .htaccess method have higher priority than what is set for the virtualhost, or the subfolder within the DocumentRoot of the virtual host.

Testing

Create a file named ‘i.php’ in the locations with the different PHP versions (can be different virtualhosts or folders)

<?php
phpversion();
?>

Access these locations on the virtualhosts or their directory locations to verify that they are using different PHP versions.

References

Most useful resource for this guide
How To Run Multiple PHP Versions on One Server Using Apache and PHP-FPM on Ubuntu 18.04 (DigitalOcean Community)