code: public static function clearCache($key = null)
      {
          if (null === $key) {
              self::$cache = [];
  
              return;
          }
  
          if (self::hasCache($key)) {
              unset(self::$cache[$key]);
          }
      }
docs: Clear cache - one position or all of them.
  
  @param null|string $key position to clear, when null clear all