ftp_nb_fget

<?php

// open some file for writing
$file = 'index.php';
$fp = fopen($file, 'w');

$ftp = ftp_connect($ftp_server);

$login_result = ftp_login($ftp, $ftp_user_name, $ftp_user_pass);

// Initiate the download
$ret = ftp_nb_fget($ftp, $fp, $file, FTP_BINARY);
while ($ret == FTP_MOREDATA) {

   // Do whatever you want
   echo ".";

   // Continue downloading...
   $ret = ftp_nb_continue($ftp);
}
if ($ret != FTP_FINISHED) {
   echo "There was an error downloading the file...";
   exit(1);
}

// close filepointer
fclose($fp);
?>