Spreadsheets, driven by code.

<?php

require 'vendor/autoload.php';

use PhpOffice\PhpSpreadsheet\Spreadsheet;
use PhpOffice\PhpSpreadsheet\Writer\Xlsx;

// One workbook per region. In a real app this is whatever your query returns.
$reports = [
    'north' => [
        ['Rep', 'Units', 'Revenue'],
        ['Alice', 120, 4800],
        ['Bob', 90, 3600],
    ],
    'south' => [
        ['Rep', 'Units', 'Revenue'],
        ['Carol', 150, 6000],
        ['Dan', 70, 2800],
    ],
    'east' => [
        ['Rep', 'Units', 'Revenue'],
        ['Erin', 200, 8000],
    ],
];

// 1. A scratch directory used by this request and nobody else.
$workDir = sys_get_temp_dir() . '/reports-' . bin2hex(random_bytes(8));

if (!mkdir($workDir) && !is_dir($workDir)) {
    exit('Could not create a working directory.');
}

$zipPath = $workDir . '/reports.zip';
$zip = new ZipArchive();

if ($zip->open($zipPath, ZipArchive::CREATE | ZipArchive::OVERWRITE) !== true) {
    exit('Could not create the archive.');
}

// 2. Build each workbook, save it, then add it to the archive.
foreach ($reports as $region => $rows) {
    $spreadsheet = new Spreadsheet();
    $worksheet = $spreadsheet->getActiveSheet();
    $worksheet->setTitle(ucfirst($region));
    $worksheet->fromArray($rows, null, 'A1');

    $memberName = $region . '-sales.xlsx';
    $filePath = $workDir . '/' . $memberName;

    (new Xlsx($spreadsheet))->save($filePath);

    // addFile() records a path, it does not copy the bytes yet. The file must
    // still exist when close() runs.
    $zip->addFile($filePath, $memberName);

    // Release the workbook before building the next one.
    $spreadsheet->disconnectWorksheets();
    unset($spreadsheet, $worksheet);
}

// 3. close() is what actually writes the archive. Before this line reports.zip
//    is incomplete, so nothing may stream it yet.
$zip->close();

// 4. Send it. The archive is a real file, so its size is known up front and the
//    browser can show a true progress bar.
header('Content-Type: application/zip');
header('Content-Disposition: attachment; filename="reports.zip"');
header('Content-Length: ' . filesize($zipPath));
header('Cache-Control: max-age=0');

while (ob_get_level() > 0) {
    ob_end_clean();
}

readfile($zipPath);

// 5. Clean up. Nothing is left behind in the temp directory.
foreach (glob($workDir . '/*') as $leftover) {
    unlink($leftover);
}
rmdir($workDir);

exit;