(PHP 4 >= 4.3.0, PHP 5, PHP 7)
ftp_nb_get — Retrieves a file from the FTP server and writes it to a local file (non-blocking)
$ftp_stream
   , string $local_file
   , string $remote_file
   , int $mode
   [, int $resumepos = 0
  ] )ftp_nb_get() retrieves a remote file from the FTP server, and saves it into a local file.
The difference between this function and ftp_get() 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.
local_fileThe local file path (will be overwritten if the file already exists).
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_get() example
<?php
// Initate the download
$ret = ftp_nb_get($my_connection, "test", "README", FTP_BINARY);
while ($ret == FTP_MOREDATA) {
   
   // Do whatever you want
   echo ".";
   // Continue downloading...
   $ret = ftp_nb_continue($my_connection);
}
if ($ret != FTP_FINISHED) {
   echo "There was an error downloading the file...";
   exit(1);
}
?>
Example #2 Resuming a download with ftp_nb_get()
<?php
// Initate 
$ret = ftp_nb_get($my_connection, "test", "README", FTP_BINARY, 
                      filesize("test"));
// OR: $ret = ftp_nb_get($my_connection, "test", "README", 
//                           FTP_BINARY, FTP_AUTORESUME);
while ($ret == FTP_MOREDATA) {
   
   // Do whatever you want
   echo ".";
   // Continue downloading...
   $ret = ftp_nb_continue($my_connection);
}
if ($ret != FTP_FINISHED) {
   echo "There was an error downloading the file...";
   exit(1);
}
?>
Example #3 Resuming a download at position 100 to a new file with ftp_nb_get()
<?php
// Disable Autoseek
ftp_set_option($my_connection, FTP_AUTOSEEK, false);
// Initiate
$ret = ftp_nb_get($my_connection, "newfile", "README", FTP_BINARY, 100);
while ($ret == FTP_MOREDATA) {
   /* ... */
   
   // Continue downloading...
   $ret = ftp_nb_continue($my_connection);
}
?>
   In the example above, newfile is 100 bytes smaller
   than README on the FTP server because we started
   reading at offset 100. If we didn't disable 
   FTP_AUTOSEEK, the first 100 bytes of
   newfile would be '\0'.