How to create downloader in CakePHP3

1.  How to create uploader in CakePHP3
2.  How to create downloader in CakePHP3
3.  How to create PDF downloader in CakePHP3

Add these functions in the "customers" controller:
(Please note that you need to add these lines also at the top of the controller:
use Cake\I18n\Date;
use ZipArchive;
use Cake\Network\Exception\BadRequestException;
)

    private function getExtension($filename){
        $temp = explode('.', $filename);
        $extension = strtolower(end($temp));
        return $extension;
    }

    public function zipDownload(){
        if ($this->request->is('ajax')) {
            $layout = 'ajax';
            $this->autoRender = false;
            $dir = WWW_ROOT . 'temp/zip/';
            $copyFromDir = WWW_ROOT.'uploads/customers/';
            $files = $this->request->data['files'];
            try {
                echo $this->createZip($dir, $copyFromDir, $files);
            } catch (BadRequestException $e) {
                throw $e;
            }
        }
    }

    function createZip($uploadToDir = null, $copyFromDir = null, $files = null, $containsYearmonth = false){
        if(!file_exists($uploadToDir)){
            if(!mkdir($uploadToDir, 0755, true)){
                throw new BadRequestException('Can not mkdir.');
            }
            chmod($uploadToDir, 0755);
        }
        try {
            $zip = new ZipArchive();
            $zip_file_name = sha1(time() . rand()).".zip";
            $zip_name = $uploadToDir.$zip_file_name; // Zip name
            $zip->open($zip_name,  ZipArchive::CREATE);
            foreach ($files as $file) {
                $path = $copyFromDir.$file;
                if(file_exists($path)){
                    $zip->addFromString(basename($path), file_get_contents($path));
                }
                else{
                    echo "file does not exist";
                }
            }
            $zip->close();
            return $zip_file_name;
        } catch (BadRequestException $e) {
            throw $e;
        }
    }

And change "index.ctp" in the "Template" folder like this:

<?php
/**
 * @var \App\View\AppView $this
 * @var \App\Model\Entity\Customer[]|\Cake\Collection\CollectionInterface $customers
 */
?>
<nav class="large-3 medium-4 columns" id="actions-sidebar">
    <ul class="side-nav">
        <li class="heading"><?= __('Actions') ?></li>
        <li><?= $this->Html->link(__('New Customer'), ['action' => 'add']) ?></li>
    </ul>
</nav>
<div class="customers index large-9 medium-8 columns content">
    <h3><?= __('Customers') ?></h3>
    <div><a href="#" onclick="downloadDocuments()">Download As Zip</a></div>
    <table cellpadding="0" cellspacing="0">
        <thead>
            <tr>
                <th scope="col" class="text-center" style="vertical-align: middle;"><?= $this->Form->checkbox('check_all', ['id'=>'check_all']);?></th>
                <th scope="col"><?= $this->Paginator->sort('id') ?></th>
                <th scope="col"><?= $this->Paginator->sort('name') ?></th>
                <th scope="col"><?= $this->Paginator->sort('file_location', 'File') ?></th>
                <th scope="col"><?= $this->Paginator->sort('created') ?></th>
                <th scope="col"><?= $this->Paginator->sort('modified') ?></th>
                <th scope="col" class="actions"><?= __('Actions') ?></th>
            </tr>
        </thead>
        <tbody>
            <?php foreach ($customers as $customer): ?>
            <tr>
                <?php $temp = explode('.', $customer->file_location); $extension = strtolower(end($temp)); ?>
                <?php $file = ['file_location' => $customer->file_location, 'file_name'=>$customer->name, 'extension'=>$extension] ?>
                <td class="text-center"><?= $this->Form->checkbox('checkbox',['name'=>'check_box', 'value'=>h(json_encode($file))]) ?></td>
                <td><?= $this->Number->format($customer->id) ?></td>
                <td><?= h($customer->name) ?></td>
                <td><img src="/cake/uploads/customers/<?= $customer->file_location ?>" width="100" /></td>
                <td><?= h($customer->created) ?></td>
                <td><?= h($customer->modified) ?></td>
                <td class="actions">
                    <?= $this->Html->link(__('View'), ['action' => 'view', $customer->id]) ?>
                    <?= $this->Html->link(__('Edit'), ['action' => 'edit', $customer->id]) ?>
                    <?= $this->Form->postLink(__('Delete'), ['action' => 'delete', $customer->id], ['confirm' => __('Are you sure you want to delete # {0}?', $customer->id)]) ?>
                </td>
            </tr>
            <?php endforeach; ?>
        </tbody>
    </table>
    <div class="paginator">
        <ul class="pagination">
            <?= $this->Paginator->first('<< ' . __('first')) ?>
            <?= $this->Paginator->prev('< ' . __('previous')) ?>
            <?= $this->Paginator->numbers() ?>
            <?= $this->Paginator->next(__('next') . ' >') ?>
            <?= $this->Paginator->last(__('last') . ' >>') ?>
        </ul>
        <p><?= $this->Paginator->counter(['format' => __('Page {{page}} of {{pages}}, showing {{current}} record(s) out of {{count}} total')]) ?></p>
    </div>
</div>
<script
  src="https://code.jquery.com/jquery-3.2.1.min.js"
  integrity="sha256-hwg4gsxgFZhOsEEamdOYGBf13FyQuiTwlAQgxVSNgt4="
  crossorigin="anonymous"></script>
<script>
    $(document).ready(function(){
        $('#check_all').on('click', function() {
            $('input:checkbox[name=check_box]').prop('checked', this.checked);
        });
    });
    function getSelectedDocuments(pdfonly){
        if (pdfonly === undefined) {
            pdfonly = false;
        }
        var checkeditems = [];
        var files_notexcel = [];
        $("input[type=checkbox][name=check_box]").each(function(){
            if (this.checked) {
                var value = JSON.parse($(this).val());
                if(pdfonly === true){
                    var extension = value.extension.toLowerCase();
                    if($.inArray(extension, ['xlsx','xlsm','xls']) !== -1){
                        checkeditems.push(value.file_location);
                    } else {
                        var name = value.file_location.split("/");
                        files_notexcel.push(name[name.length - 1]);
                    }
                } else {
                    checkeditems.push(value.file_location);
                }
            }
        });
        var files = {"checkeditems" : checkeditems, "files_notexcel" : files_notexcel};
        return files;
    }
    function download(url){
        var link=document.createElement('a');
        document.body.appendChild(link);
        link.href=url;
        link.download = 'download';
        link.click();
        link.parentNode.removeChild(link);
    }
    function downloadDocuments(){
        var checkeditems = getSelectedDocuments();
        checkeditems = checkeditems.checkeditems;
        if(checkeditems.length < 1 || checkeditems === undefined){
            alert('Please select at least one document.');
        } else {
            var csrfToken = $('[name=_csrfToken]').val();
            $.ajax({
                type: "POST",
                url: '<?= $this->Url->build(array("controller" => "Customers", "action" => "zipDownload")); ?>',
                data: {'files' : checkeditems},
                beforeSend: function(xhr){
                    xhr.setRequestHeader('X-CSRF-Token', csrfToken);
                },
                success: function(data){
                    var url = '/cake/temp/zip/'+data;
                    console.log(url);
                    download(url);
                }
            });
        }
    }
</script>

And you can download the files in index.ctp: