This one is quite simple to be honest. Assuming you have Redis already installed and set up correctly you can just change your php-fpm pool definition to include:

php_value[session.save_handler] = redis
php_value[session.save_path]    = "tcp://"

If you’re using password for logging into redis you would specify it like:

php_value[session.save_path]    = "tcp://"

Restart php-fpm after that:

service php-fpm restart

and you should now see PHP saving sessions within Redis service:> keys PHPREDIS_SESSION*