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 turned on (either Smarty::CACHING_LIFETIME_CURRENT or Smarty::CACHING_LIFETIME_SAVED) for
$cache_lifetime to have any purpose.
$cache_lifetime 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 = Smarty::CACHING_OFF).
If you want to give certain templates their own cache lifetime,
do this by setting
$caching = Smarty::CACHING_LIFETIME_SAVED,
$cache_lifetime to a unique value just
enabled, the cache files will be regenerated every time, effectively
disabling caching. You can clear all the cache files with the
function, or individual cache files (or groups) with the