Trying to zip and download multiple file directories PHP

I'm new to coding and i am trying to figure out how to zip and download multiple folders after selecting them via a series of check boxes.

However i keep getting the " ZipArchive::close(): Can't open file: Permission denied on line 21" whenever i try to run the code.

<?php

if (isset($_POST['dwnld'])) {
  //create ana array
  $ar=array();

  $checked=$_POST['dwnld'];

  for ($i=0; $i <count($checked) ; $i++) {
    $f="../Docs/".$checked[$i];
    $ar[$i]=$f;
  }

  $files = $ar;
  $zipname = 'file.zip';
  $zip = new ZipArchive;
  $zip->open($zipname, ZipArchive::CREATE);
  foreach ($files as $file) {
    $zip->addFile($file);
  }
  $zip->close();
}

?>

Any advise is welcomed.