code: public function fetchOffsetKey($num)
      {
          // Keys for resources are always strings. If given a numerical offset, 
          // that offset needs to be interpreted to determine the actual property's name
          // at that offset.
          return isset($this->contentKeys[$num]) ? $this->contentKeys[$num] : null;
      }
docs: Returns the offset name (string) of a resource for a given numerical offset.
  
  @param num A numerical offset.