+ * Example usage (key that is expensive with too many DB dependencies for "check keys"):
+ * @code
+ * $catToys = $cache->getWithSetCallback(
+ * // Key to store the cached value under
+ * $cache->makeKey( 'cat-toys', $catId ),
+ * // Time-to-live (seconds)
+ * $cache::TTL_HOUR,
+ * // Function that derives the new key value
+ * function ( $oldValue, &$ttl, array &$setOpts ) {
+ * // Determine new value from the DB
+ * $dbr = wfGetDB( DB_REPLICA );
+ * // Account for any snapshot/replica DB lag
+ * $setOpts += Database::getCacheSetOptions( $dbr );
+ *
+ * return CatToys::newFromResults( $dbr->select( ... ) );
+ * },
+ * [
+ * // Get the highest timestamp of any of the cat's toys
+ * 'touchedCallback' => function ( $value ) use ( $catId ) {
+ * $dbr = wfGetDB( DB_REPLICA );
+ * $ts = $dbr->selectField( 'cat_toys', 'MAX(ct_touched)', ... );
+ *
+ * return wfTimestampOrNull( TS_UNIX, $ts );
+ * },
+ * // Avoid DB queries for repeated access
+ * 'pcTTL' => $cache::TTL_PROC_SHORT
+ * ]
+ * );
+ * @endcode
+ *