Note: This is a modified copy of https://www.howtoforge.com/tutorial/how-to-install-php-7-for-ispconfig-3-from-debian-packages-on-debian-8-and-9/, which is more detailed and gives better explanations.

However, sury.org breaks systems which do not use systemd. I use the old standby, sysV init, which is available on Devuan Linux due to problems I have with systemd.

First, set up the repository for TRD Networks as described in sury.org breaks Devuan and install the versions of PHP you want. You can use the repository from TRD Networks on systemd based installs, but you can not use sury.org on non-systemd based systems.

Second, add the PHP Versions you want to your ISPConfig3 install. Following are just copied from their site. I recommend you use the Howto Forge site unless you already know what you're doing, as they give better explanations.

PHP 7.4
Path to the PHP FastCGI binary: /usr/bin/php-cgi7.4
Path to the php.ini directory: /etc/php/7.4/cgi/php.ini

Path to the PHP-FPM init script: php7.4-fpm
Path to the php.ini directory: /etc/php/7.4/fpm/php.ini
Path to the PHP-FPM pool directory: /etc/php/7.4/fpm/pool.d

PHP 8.0
Path to the PHP FastCGI binary: /usr/bin/php-cgi8.0
Path to the php.ini directory: /etc/php/8.0/cgi/

Path to the PHP-FPM init script: php8.0-fpm
Path to the php.ini directory: /etc/php/8.0/fpm/
Path to the PHP-FPM pool directory: /etc/php/8.0/fpm/pool.d

PHP 8.1
Path to the PHP FastCGI binary: /usr/bin/php-cgi8.1
Path to the php.ini directory: /etc/php/8.1/cgi/

Path to the PHP-FPM init script: php8.1-fpm
Path to the php.ini directory: /etc/php/8.1/fpm/
Path to the PHP-FPM pool directory: /etc/php/8.1/fpm/pool.d