$cache_lifetime
This is the length of time in seconds that a template cache is valid. Once this time has expired, the cache will be regenerated. $caching must be set to "true" for $cache_lifetime to have any purpose. A value of -1 will force the cache to never expire. A value of 0 will cause the cache to always regenerate (good for testing only, to disable caching a more efficient method is to set $caching = false.)
If $force_compile is enabled, the cache files will be regenerated every time, effectively disabling caching. You can clear all the cache files with the clear_all_cache() function, or individual cache files (or groups) with the clear_cache() function.
Technical Note: If you want to give certain templates their own cache lifetime, you could do this by setting $caching = 2, then set $cache_lifetime to a unique value just before calling display() or fetch().
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