Using pecl:

 pecl install memcache-3.0.4

or manually:

 wget http://pecl.php.net/get/memcache-3.0.4.tgz
 tar xvfz memcache-3.0.4.tgz
 cd memcache-3.0.4
 phpize
 ./configure
 make
 make install

Finally, we need to activate the module in php.ini. I normally prefer to create a new file for this memcache.ini inside the include directory of the php build (for ex. in debian this is under /etc/php5/conf.d/memcache.ini) like this:

extension=memcache.so
memcache.allow_failover = 1
memcache.redundancy = 1
memcache.session_redundancy = 2

To use memcached to store the php sessions we will have to edit php.ini and replace the default file handler settings with something like:

 ; Use memcache as a session handler
 session.save_handler = memcache
 ; Use a comma separated list of server urls to use for storage:
 session.save_path="udp://<memcache_server>:11211?persistent=1&weight=1&timeout=1&retry_interval=15"

or if we don’t want to use this serverwide, we can just define it inside a php block like this:

 $session_save_path = "tcp://<memcache_server1>:11211?persistent=1&weight=1&timeout=1&retry_interval=15,tcp://<memcache_server2>:11211";
 ini_set('session.save_handler', 'memcache');
 ini_set('session.save_path', $session_save_path);

Note: as you can see I used in the above example tcp and udp also. Please be sure that your memcached server has udp support enabled if you want to use that. Also ensure that the web server can connect to the memcache server/port (use proper firewall rules to allow this) in order for this to work.

After restarting apache, it will start using memcache to store the php sessions. If redundancy is needed (why not?) we will probably want to use the internal php memcache support to save the sessions to more servers, or if you prefer you can use an external solution to replicate the memcached server data - repcached.