Till Klampaeckel has shown off one of the "best kept secrets" about the APC caching tool that not many people seem to use - getting a key's expiration time that can be useful to tell other applications how long the data will be good for.
APC offers a bunch of very useful features — foremost a realpath cache and an opcode cache. However, my favorite is neither: it's being able to cache data in shared memory. How so? Simple: use apc_store() and apc_fetch() to persist data between requests. The other day, I wanted use a key's expiration date to send the appropriate headers (Expires and Last-Modified) to the client, but it didn't seem like APC supports this out of the box yet.
He includes a quick bit of sample code that defines an "apc_exire" function that grabs the expiration information as returned by apc_cache_info - the "ttl" and "creation_time" values.