PHP script not working on a hosted system

I have the following script that works on a local system, but not on a web-hosted system (where it needs to be). Locally, it downloads the file with no issues. When hosted, I get either zero- or low-byte files instead of the entire file.

<?php

$username = "user";
$password = "password";
$server = "ftp.domain.com";
$path = $_GET['p'];
$filename = $_GET['f'];

$file_full = $path."/".$filename;

$file_path = "ftp://".$username.":".$password."@".$server."/".$file_full;
$fsize = filesize($file_path); 

header("Pragma: public");
header("Expires: 0");
header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
header("Cache-Control: public");
header("Content-Description: File Transfer"); 
//header("Content-Type: $mtype");
header("Content-Type: application/octet-stream");
//header("Content-Disposition: attachment; filename=\"$asfname\""); */
header("Content-Disposition: attachment; filename=\"$filename\"");
header("Content-Transfer-Encoding: binary"); 
header("Content-Length: " . $fsize);


// download
// @readfile($file_path);
$file = @fopen($file_path,"rb");
if ($file) {
  while(!feof($file)) {
    print(fread($file, 1024*8));
    flush();
    if (connection_status()!=0) {
      @fclose($file);
      die();
    }
  }
  @fclose($file);
}

?>

My host provider is Hostmonster (if that helps at all).

In case you're wondering, I know I can just download links via FTP, but I need to force the user to choose whether to "Save As" or "Open", hence the script.