ftp_get() retrieves remote_file from the FTP server, and
saves it to local_file
locally. The transfer mode
specified must be either
FTP_ASCII or
FTP_BINARY.
注: The resumepos parameter was added in PHP 4.3.0.
成功回傳TRUE失敗回傳FALSE。
See also ftp_fget() and
ftp_async_get().