public function getParentFolder($force = FALSE) {
		
		if(!isset($this->parentFolder) || $force) {
			
			$parentPath = realpath($this->path . '..');
			
			// flag parentFolder property, when $this is already the root folder
			
			if($parentPath === $this->path) {
				$this->parentFolder = FALSE;
			}

			else {
				$this->parentFolder = self::getInstance($parentPath);
			}

		}

		// return NULL (instead of FALSE) when there is no parent folder

		return $this->parentFolder ?: NULL;
		
	}