PHP Download Multiple Files in Zip folder

Question

I'm trying to download multiple files from the server in a zipped folder. Here is the code I have thus far:

HTML

<form method="post">
    <input type="checkbox" name="checked[]" value="<?php echo $path; ?>">
    <input type="submit" name="download" value="Download Selected">
</form>

PHP:

if(isset($_POST['download'])){
    $files = $_POST['checked'];
    $zip = new ZipArchive();
    $zip_name = time().".zip"; // Zip name
    $zip->open($zip_name,  ZipArchive::CREATE);
    $full = wp_upload_dir();
    $base = $full['baseurl'] .'/';


foreach ($files as $file) {
  echo $full_path = $file;
  echo $_SERVER['DOCUMENT_ROOT'] . $path;
  if(file_exists($_SERVER['DOCUMENT_ROOT'].'/upload/folder/'.$full_path)){
      $zip->addFromString(basename($base . $full_path),  file_get_contents($base . $full_path));  
  }
  else{
   echo"file does not exist";
  }
}
$zip->close();
}