How to install PHP 7.1, 7.2 and 5.6 as PHP-FPM & FastCGI for ISPConfig 3 on Debian 9

Debian 9 ships with PHP 7.0 as default PHP version. Major PHP versions are not 100% compatible with each other, so a website might require a newer or older PHP version to work. ISPConfig supports it to use multiple PHP versions on the same server, the PHP version can be selected for each website individually in the website settings. This tutorial shows how to install PHP 7.1, 7.2, and 5.6 as FPM and FCGI mode alongside PHP 7.0 on a Debian 9 server. The additional PHP versions are installed in the /opt folder, so their installation does nit affect the default PHP version

https://www.howtoforge.com/tutorial/how-to-install-php-5.6-and-7.1-on-debian-9-stretch/