head of to the System tab (cog icon) and click ‘Additional PHP Versions’ from the menu on the left.

hit the ‘Add new PHP button’ then select the server you’ll be adding this new version to from the drop-down.

leave ‘Client’ blank and enter PHP name as PHP 8.0

on to the FastCGI tab and enter the following

Path to the PHP FastCGI binary: /usr/bin/php-cgi8.0

Path to the php.ini directory: /etc/php/8.0/cgi/php.ini

and now to the PHP-FPM tab and enter

Path to the PHP-FPM init script: /usr/bin/php8.0

Path to the php.ini directory: /etc/php/8.0/fpm/php.ini

Path to the PHP-FPM pool directory: /etc/php/8.0/fpm/pool.d

and that’s it. hit save and ISPConfig will apply the settings. PHP 8.0 can now be chosen in the individual sites’ settings.

use at your own risk etc.