Memcache Installation Issues When Running PHP APC

When running PHP APC and installing the Memcache Drupal Module (even with the site offline) you will get some nice PHP errors with a botched installation and non of the Memcache admin menus showing up etc.

[Thu Nov 24 23:01:05 2011] [error] [client xx.xx.xx.xx] PHP Fatal error: require_once(): Cannot redeclare class queryconditioninterface in /var/www/xxx.xxx/xxx/htdocs/includes/registry.inc on line 32, referer: http://xxx.xxx.xxx/admin/modules

This means...
(1) Disabled APC via apc.ini edit: apc.enabled="0". Restart Apache.
(2) Reinstall Memcache Drupal Module with site in offline mode.
(3) Add Memcache specific config to settings.php:

# Memcache Configuration
$conf['cache_backends'][] = 'sites/all/modules/memcache/memcache.inc';
$conf['cache_class_cache_form'] = 'DrupalDatabaseCache';
$conf['memcache_key_prefix'] = 'xxx';

Assuming you are running default localhost:port Memcache daemon on the same server, the above is all you need.

(4) Renable PHP APC. Restart Apache.
(5) Go have a beer.

The PHP APC issue should be added into documentation somewhere !