* (which will be false if not present), and takes the arguments:
* (this BagOStuff, cache key, current value, TTL).
* The TTL parameter is reference set to $exptime. It can be overriden in the callback.
+ * If the callback returns false, then the current value will be unchanged (including TTL).
*
* @param string $key
* @param callable $callback Callback method to be executed