Took me forever to work out but this will work if you get the paths right!
<?php
function ftp_copy($file) {
global $conn_id;
$ftp_root = '/public_html/';
$site_root = '/home/usr/public_html/';
return ftp_put($conn_id, $ftp_root . $file, $site_root . $file, FTP_BINARY);
}
?>
Login to your site using an FTP client to retrieve the $ftp_root
Use something like getcwd() to get the $site root