How to install PHP 5.6 and 7.0 – 8.4 with PHP-FPM and FastCGI mode for ISPConfig 3.2 with apt on Debian 11 to 12
In this guide we will take you through installing additional PHP versions (5.6, 7.0, 7.1, 7.2, 7.3, 7.4, 8.1, 8.2, 8.3, and 8.4) on a Debian server with ISPConfig.
Source: Howtoforge Linux Howtos und Tutorials