When using ISPConfig, by default, you only have the main PHP version for your distribution. This guide will take you through installing additional PHP versions (5.6, 7.0, 7.1, 7.2, 7.3, 7.4, 8.1, and 8.2) on a Ubuntu server with ISPConfig.

Source: LXer – How to install PHP 5.6 and 7.0 – 8.2 with PHP-FPM and FastCGI mode for ISPConfig 3.2