Returns the group ID of the owner of the file, or FALSE in case of an error. The group ID is returned in numerical format, use posix_getgrgid() to resolve it to a group name.
The results of this function are cached. See clearstatcache() for more details.
Note: This function does not work on Windows systems
This function will not work on remote files; the file to be examined must be accessible via the server's filesystem.