A function that hooks into WordPress and, when a $_GET request with a key of "download" and a value that contains a Base64 encoded array of semi-colon separated file URLS is passed, generates the ZIP on the fly and downloads it

[WP] Force download headers

add_action('wp', 'handle_all_file_downloads');

function handle_all_file_downloads() {
	if (isset($_GET) && isset($_GET['download'])) {
		$urls = base64_decode($_GET['download']);

		if ($urls) {
			$urls = (strpos($urls, ';') !== false) ? explode(';', $urls) : array($urls);
			$path = wp_upload_dir();
			$zip = new ZipArchive();
			$zip_name = uniqid('dwwindsor-download-');
			$zip_path = $path['basedir'] . '/' . $zip_name . '.zip';

			if (file_exists($zip_path)) {
				unlink($zip_path);
			}

			if ($zip->open($zip_path, ZIPARCHIVE::CREATE) === true) {
				$zip->addEmptyDir($zip_name);

				foreach ($urls as $url) {
					$parameters = explode('/', $url);
					$zip->addFromString($zip_name . '/' . end($parameters), file_get_contents($url));
				}

				$zip->close();

				header('Content-disposition: attachment; filename=' . $zip_name . '.zip');
				header('Content-type: application/zip');

				@readfile($zip_path);
				unlink($zip_path);

				exit;
			}
		}
	}
}