Mark Jaquith

Someone correct me if my understanding is inaccurate.

APC (or X-Cache) does what is called as PHP opcode caching. Normally, PHP reads the .php files and “compiles” them on the fly. Every time. That is obviously not super efficient. APC and X-Cache store that “compiled” opcode and hang on to it, which gives you a nice PHP speed boost.

APC also provides a single-server in-memory object cache. Key/value store. That doesn’t automatically give you anything in WordPress. For that, you need an APC object cache backend, which is a drop-in file called object-cache.php that WordPress will use to interface with APC. Now WordPress just uses this for its object storage… not for HTML output caching. For that, you’d need something like Batcache, which can use the Memcached or APC object cache backends to do persistent HTML output caching. Batcache is another drop-in file called advanced-cache.php, which WordPress utilizes to do an early “hand off” (so the caching can happen before WordPress fully loads).

But Batcache by itself does not tell posts to refresh when they’re changed. For that, you need the Batcache plugin, which tells Batcache (the advanced cache drop-in) to tell the APC object cache backend to tell the APC persistent cache to flush that page out.

1. Opcode caching (APC or X-Cache) — speeds up PHP in general.
2. Object cache backend drop-in (for APC or Memcached) — gives WordPress persistent storage for objects.
3. Batcache (advanced caching drop-in) — caches HTML using the WordPress object cache (#2).
4. Batcache helper plugin — helps Batcache (#3) tell the object cache backend (#2) to flush things when posts change.

This will be fixed in the next version. But I wouldn’t advise running your site with PHP notices being displayed! You’re going to have a lot more issues with other plugins if you do that.