Download all files as zip when button is clicked - php

Question

I can't understand how my button should look and trigger download files as zip. On the page I can add files in some sort of cart and when I finish with file adding I want to download them like zip.
So what I have so far is this
<?php
include 'database.inc.php';
$pdo = Database::connect();
$pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
if(isset($_POST['total_cart_items']))
{
echo count($_SESSION['itemid']);
exit();
}
if(isset($_POST['item_id']))
{
if (!in_array($_POST['item_id'], $_SESSION['itemid'])) $_SESSION['itemid'][]=$_POST['item_id'];
echo count($_SESSION['itemid']);
exit();
}
if(! isset($_POST['showcart'])) exit;
foreach ($_SESSION['itemid'] as $i):
$sql = "SELECT * FROM document_upload WHERE upload_id = :id";
$result = $pdo->prepare($sql);
$result->bindParam(":id", $i);
$result->execute();
$resArray = $result->fetchAll();
foreach ( $resArray as $row ):?>
<div class="cart_items">
<a style="text-align:center;" href=""><p><?=$row["upload_title"]?> - <?=$row["upload_description"]?></p></a>
</div>
<?php endforeach;?>
<?php endforeach; ?>
<a class="btn btn-info btn-sm pull-right" style="margin: 10px;" href="">Download All Files</a>
So do I need another file which will load when I click on Download All Files or?
EDIT: I found this. The problem is how to pass all the files for download to this function?
session_start();
$files = array();
$valid_files = array();
if(is_array($files)) {
foreach($files as $file) {
if(file_exists($file)) {
$valid_files[] = $file;
}
}
}
if(count($valid_files > 0)){
$zip = new ZipArchive();
$zip_name = "zipfile.zip";
if($zip->open($zip_name, ZIPARCHIVE::CREATE)!==TRUE){
$error .= "* Sorry ZIP creation failed at this time";
}
foreach($valid_files as $file){
$zip->addFile($file);
}
$zip->close();
if(file_exists($zip_name)){
// force to download the zip
header("Pragma: public");
header("Expires: 0");
header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
header("Cache-Control: private",false);
header('Content-type: application/zip');
header('Content-Disposition: attachment; filename="'.$zip_name.'"');
readfile($zip_name);
// remove zip file from temp path
unlink($zip_name);
}
} else {
echo "No valid files to zip";
exit;
}