(PHP 4 = 4.3.0)
ftp_nb_fget() retrieves remote_file from the FTP
server, and writes it to the given file pointer, handle. The transfer mode specified must be either FTP_ASCII or FTP_BINARY. The difference between
this function and the ftp_fget() is that this function
retrieves the file asynchronously, so your program can
perform other operations while the file is being
downloaded.
成功回傳TRUE失敗回傳FALSE。
See also ftp_nb_get().