* - Cached or regenerated value version number or null if not versioned
* - Timestamp of the current cached value at the key or null if there is no value
* @note Callable type hints are not used to avoid class-autoloading
- * @suppress PhanTypeArraySuspicious
*/
private function fetchOrRegenerate( $key, $ttl, $callback, array $opts ) {
$checkKeys = $opts['checkKeys'] ?? [];