PHPOffice/PhpSpreadsheet

@dack94 I did exactly what you are talking about a long time ago, and it's a surprisingly trick thing to do. I would try to avoid it actually. One serious limitation with normal ajax thing is the browser limitation of 2 connection to a given server. So in some condition, your ajax polling might not actually start before the excel file is done generating. Anyway here is my solution that you will have to adapt.

Utility Class:

<?php

namespace Application\Utility;

/**
 * This class implements ways to know when a long process is running on
 * the server and when the said process is complete.
 *
 * This file *MUST NOT* be part of Zend code because it has to be accessible concurrently
 * to any process that may occur at the same time by another HTTP request. Hence we
 * cannot use session and cannot use ACL security system neither. It must stay totally
 * separated from all other code.
 */
abstract class Progress
{
    private static $currentIdProgress = '';

    /**
     * Returns the path for the path for the lock file for the specified ID of progress
     *
     * @param string $id
     *
     * @return string
     */
    private static function getLockFilePath(string $id): string
    {
        // Here we use MD5 to prevent security hole to access filesystem with '.' or '/' characters in $id
        return __DIR__ . '/data/tmp/progress_' . md5($id) . '.lock';
    }

    /**
     * Notify the beginning of a long process
     *
     * @param string $id
     */
    public static function start(string $id): void
    {
        self::$currentIdProgress = $id;
        touch(self::getLockFilePath($id));
    }

    /**
     * Notify the end of a long process
     */
    public static function end(): void
    {
        $path = self::getLockFilePath(self::$currentIdProgress);
        if (file_exists($path)) {
            unlink($path);
        }
    }

    /**
     * Allow to poll the status of a progress by its ID
     *
     * @param string $id id of progress
     *
     * @return string current status of progress
     */
    public static function status(string $id): string
    {
        if ($id && file_exists(self::getLockFilePath($id))) {
            return $id . ' in progress ...';
        }

        return '';
    }
}

Then your javascript would start the download with an arbitrary "process id". Any string generated in JS. When starting to generate the excel file and you receive that id in PHP, you would call:

Progress::start($_REQUEST['id']);
$spreadsheet = createMySpreadsheet();
Progress::end();
sendSpreadsheetToBrowser($spreadsheet);

In parallel the JS would start to poll a different endpoint. That endpoint MUST be completely separate from your application and it MUST NOT use PHP session at all. In that new endpoint you use:

<?php

/**
 * This file *MUST NOT* be part of Zend code because it has to be accessible concurrently
 * to any process that may occur at the same time by another HTTP request. Hence we
 * cannot use session and cannot use ACL security system neither. It must stay totally
 * separated from all other code.
 */
require_once __DIR__ . '/../vendor/autoload.php';
echo \Application\Utility\Progress::status($_REQUEST['id']);

The JS can then show an non-deterministric progress bar, until the endpoint says it's over