There is an existing smarty memcache cache handler function out there,
however examples I've seen out there are incomplete in that they only
support evicting items from the cache using an exact key. However,
GitPHP takes advantage of some of the more advanced cache expiration
features of smarty, such as using cache groups and expiration based on
cache age.
Therefore this is an implementation done by me from scratch to support
these extra cache expiration mechanisms. Memcache doesn't inherently
support things like iterating over stored keys and getting the age of
a stored item. Therefore, I'm using a hack, where I maintain a mapping
table of stored keys and cache ages in a fixed key, and update that key
manually and use it for cache group or age expiration.
This also provides a memcache wrapper class to be able to transparently
support both the PECL Memcache and PECL Memcached extensions using the
same functions.