PHP - Merging files with FTP
<?php

// FTP credentials
$server = HIDDEN;
$username = HIDDEN;
$password = HIDDEN;

// Connect to FTP
$connection = ftp_connect($server) or die("Failed to connect to <b>$server</b>.");

// Login to FTP
if (!@ftp_login($connection, $username, $password))
{
    echo "Failed to login to <b>$server</b> as <b>$username</b>.";
}

// Destination file (where the copied file should go)
$destination = 'final.txt';

// The file on my server that we're copying (in chunks) to $destination.
$read = 'readme.txt';

// Current chunk of $read.
$temp = 'temp.tmp';

// If the file we're trying to copy exists...
if (file_exists($read))
{
    // Set a chunk size (this is tiny, but I'm testing
    // with tiny files just to make sure it works)
    $chunk_size = 4;

    // For reading through the file we want to copy to the FTP server.
    $read_handle = fopen($read, 'r');

    // For writing the chunk to its own file.
    $temp_handle = fopen($temp, 'w+');

    // Loop through $read until we reach the end of the file.
    while (!feof($read_handle))
    {
        // Read a chunk of the file we're copying.
        $chunk = fread($read_handle, $chunk_size);

        // Write that chunk to its own file.
        fwrite($temp_handle, $chunk);

        ////////////////////////////////////////////
        ////                                    ////
        ////       NOW WHAT?? HOW DO I          ////
        ////       WRITE / APPEND THAT          ////
        ////       CHUNK TO $destination?       ////
        ////                                    ////
        ////////////////////////////////////////////
    }
}

fclose($read_handle);
fclose($temp_handle);
ftp_close($ftp_connect);
?>

<?php
// The URI of the remote file to be written to
$write = 'ftp://username1:password1@domain1.com/path/to/writeme.txt';

// The URI of the remote file to be read
$read = 'ftp://username2:password2@domain2.com/path/to/readme.txt';

if (file_exists($read)) // this will work over ftp too
{
    $chunk_size = 4;
    $read_handle = fopen($read, 'r');
    $write_handle = fopen($write, 'a');

    while (!feof($read_handle))
    {
        $chunk = fread($read_handle, $chunk_size);
        fwrite($write_handle, $chunk);
    }
}

fclose($read_handle);
fclose($write_handle);
?>