(PHP 4 >= 4.3.0, PHP 5, PHP 7)
ftp_nb_fget — Retrieves a file from the FTP server and writes it to an open file (non-blocking)
$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.
ftp_streamThe link identifier of the FTP connection.
handleAn open file pointer in which we store the data.
remote_fileThe remote file path.
mode
       The transfer mode. Must be either FTP_ASCII or
       FTP_BINARY.
      
resumeposThe position in the remote file to start downloading from.
   Returns FTP_FAILED or FTP_FINISHED
   or FTP_MOREDATA.
  
Example #1 ftp_nb_fget() example
<?php
// open some file for reading
$file = 'index.php';
$fp = fopen($file, 'w');
$conn_id = ftp_connect($ftp_server);
$login_result = ftp_login($conn_id, $ftp_user_name, $ftp_user_pass);
// Initate the download
$ret = ftp_nb_fget($conn_id, $fp, $file, FTP_BINARY);
while ($ret == FTP_MOREDATA) {
   // Do whatever you want
   echo ".";
   // Continue downloading...
   $ret = ftp_nb_continue($conn_id);
}
if ($ret != FTP_FINISHED) {
   echo "There was an error downloading the file...";
   exit(1);
}
// close filepointer
fclose($fp);
?>