PHP 5.6.x: activate APCu, installing is not enough

PHP 5.6 uses APCu as user cache, running

apt-get install php5-apcu

does only install that thing but does not configure php to actually use it. One must edit the apcu.ini file

# vim /etc/php5/cli/conf.d/20-apcu.ini

to finally activate/enable apcu. apc.enable enables it for php-fpm and apache-mod-php5 whereas apc.enable_cli also activates it for commandline-php like cronjobs also. That might be needed for OwnCloud 8.1.x or other applications.