Attempts to change the group of the file filename to group (specified by name or number). Only the superuser may change the group of a file arbitrarily; other users may change the group of a file to any group of which that user is a member.
成功回傳TRUE失敗回傳FALSE。
備注:此函數不能使用於遠端檔案 ﹐因被檢查的檔案必須通過伺服器的檔案系統存取。
備注:當安全模式開啟,PHP會檢查執行的腳本和要操作的檔案或目錄是否有相同的UID(擁有者)。