cache_TS: Cache the output of PHP script in files

This class can be used to cache the output of PHP script in files.

First it checks the cache file of a given cache key.

If the cache file exists and it has not expired, the class outputs the contents of the cache file.

If the cache file does not yet exist or it has expired, the class starts capturing the output of the script and stores it in the cache file.

The directory where the cache files are stored and the cache file expiry time are configurable parameters.