Quick guide, without explanations
Configuring Memory Caching APCu 4.0.6.1
Owncloud installation Ubuntu 14.04.3 LTS

  1. Uninstall current version
    $~: php5dismod apcu
    $~: apt-get remove php5-apcu
  2. Install APCU
    $~: wget https://launchpad.net/~ondrej/+archive/ubuntu/php5/+build/6149263/+files/php5-apcu_4.0.6-1%2Bdeb.sury.org~utopic%2B1_amd64.deb
    $~: sudo dpkg -i php5-apcu_4.0.6-1+deb.sury.org~utopic+1_amd64.deb
    $~: php5enmod apcu
  3. Small configuration
    $~: touch /etc/php5/mods-available/apcu-cli.ini
    $~: echo 'apc.enable_cli = 1' > /etc/php5/mods-available/apcu-cli.ini
    $~: php5enmod apcu-cli
    $~: service apache2 restart
  4. Open the php config in the terminal

    ownCloud is probably either installed in (/var/www/owncloud ) or (/var/www/html/owncloud)
    If owncloud is installed in (/var/www/html/owncloud )
    write in the terminal:

    $~: sudo nano /var/www/html/owncloud/config/config.php

    Before the last character

    );

    you write or copy paste from here:

    'memcache.local' => '\OC\Memcache\APCu',

    it looks like:

    'installed' => true,
'memcache.local' => '\OC\Memcache\APCu',
);

    Save and Quit with [CTRL+X] + Y

  5. Restart apache
    $~: sudo service apache2 restart
PRO TIP

If you want to upgrade PHP-APCu to 4.0.7 (PHP 5.6), just download this script and run it on your system.