int ftp_nb_fget ( resource $ftp_stream , resource $handle , string $remote_file , int $mode [, int $resumepos = 0 ] )
ftp_nb_fget() retrieves a remote file from the FTP server. The difference between this function and ftp_fget() is that this function retrieves the file asynchronously, so your program can perform other operations while the file is being downloaded.
Returns FTP_FAILED or FTP_FINISHED or FTP_MOREDATA.