install the apache module memcache

Like APC, Memcached is a high-performance, distributed memory object caching system. However, it can work together with multiple servers (unlike APC).

sudo apt-get install memcached
sudo apt-get install php-pear
sudo pecl install memcache

Check to see if Memcached is running

ps aux | grep memcached

You need to install the apache module memcache.so in your php.ini file:
extension=memcache.so

gedit /etc/php5/apache2/php.ini