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
extension=apcu.so
apc.enabled=1
apc.enable_cli=1
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.