* Fetch the value of a key from cache
*
* If passed in, $curTTL is set to the remaining TTL (current time left):
- * - a) INF; if the key exists and has no TTL
- * - b) float (>=0); if the key exists and has a TTL
- * - c) float (<0); if the key is tombstoned or expired by $checkKeys
+ * - a) INF; if the key exists, has no TTL, and is not expired by $checkKeys
+ * - b) float (>=0); if the key exists, has a TTL, and is not expired by $checkKeys
+ * - c) float (<0); if the key is tombstoned or existing but expired by $checkKeys
* - d) null; if the key does not exist and is not tombstoned
*
* If a key is tombstoned, $curTTL will reflect the time since delete().