How to install PHP 7.1 as PHP-FPM & FastCGI for ISPConfig 3.1 on Debian 8 (Jessie)

This tutorial shows how to build PHP 7.1 as a PHP-FPM and a FastCGI version on Debian 8 (Jessie) which can also be used for the multi PHP version function in the ISPConfig Hosting Control Panel. This PHP 7.1 build includes many compiled in extensions and Zend OPcache and Memcached as loadable modules.