I installed ampps 3.9 on linux mint 17 (~ ubuntu 14.04) and use with php 7.1
In the default /usr/local/ampps/www/ directory everything is ok, the php files are work correctly, but under addon domain php files are downloading instead of shown the output.
php -v output:
PHP 7.1.8 (cli) (built: Aug 17 2017 16:56:45) ( NTS )
Copyright (c) 1997-2017 The PHP Group
Zend Engine v3.1.0, Copyright (c) 1998-2017 Zend Technologies
with the ionCube PHP Loader + ionCube24 v10.4.4, Copyright (c) 2002-2020, by ionCube Ltd.
symlink created and is point to the php-7.1 folder:
sudo ln -s /usr/local/ampps/php-7.1 /usr/local/ampps/php