PHP output file on disk to browser

There is fpassthru() that should do exactly what you need. See the manual entry to read about the following example:

<?php

// open the file in a binary mode
$name = './img/ok.png';
$fp = fopen($name, 'rb');

// send the right headers
header("Content-Type: image/png");
header("Content-Length: " . filesize($name));

// dump the picture and stop the script
fpassthru($fp);
exit;

?>

See here for all of PHP's filesystem functions.

If it's a binary file you want to offer for download, you probably also want to send the right headers so the "Save as.." dialog pops up. See the 1st answer to this question for a good example on what headers to send.

Just remove the output buffering (ob_start() and the others).

Use just this:

ftp_get($conn_id, "php://output", $file, FTP_BINARY);

Though if you want to add Content-Length header, you have to query file size first using ftp_size:

$conn_id = ftp_connect("ftp.example.com");
ftp_login($conn_id, "username", "password");
ftp_pasv($conn_id, true);

$file_path = "remote/path/file.zip";
$size = ftp_size($conn_id, $file_path);

header("Content-Type: application/octet-stream");
header("Content-Disposition: attachment; filename=" . basename($file_path));
header("Content-Length: $size"); 

ftp_get($conn_id, "php://output", $file_path, FTP_BINARY);

(add error handling)

For more broad background, see:
List and download clicked file from FTP

Your implementation of move should use the rename function, which is atomic. Processes opening the file will see either the old or the new contents, there is no middle state. If a process already has opened the file it will keep having access to the old version after move.