Changes the root directory of the current process to directory. 成功回傳TRUE失敗回傳FALSE。.
注: It's not wise to use this function when running in a webserver environment, because it's not possible to reset the root directory to / again at the end of the request. This function will only function correct when you run PHP as command line too. (CLI)
注: Windows平台上沒有實作此函數。