PHP 7 FPDI Create Thumbnails of Existing PDF Document

<?php
//your variables
$filename = 'Brand-Guide.pdf';

use setasign\Fpdi\Fpdi;

require_once('fpdf/fpdf.php');
require_once('fpdi2/src/autoload.php');

$pdf = new Fpdi();

$pageCount = $pdf->setSourceFile($filename);

$width = $pdf->GetPageWidth() / 2 - 15;
$height = 0;

$_x = $x = 10;
$_y = $y = 10;

$pdf->AddPage();
for ($n = 1; $n <= $pageCount; $n++) {
    $pageId = $pdf->importPage($n);

    $size = $pdf->useImportedPage($pageId, $x, $y, $width);
    $pdf->Rect($x, $y, $size['width'], $size['height']);
    $height = max($height, $size['height']);
    if ($n % 2 == 0) {
        $y += $height + 10;
        $x = $_x;
        $height = 0;
    } else {
        $x += $width + 10;
    }

    if ($n % 4 == 0 && $n != $pageCount) {
        $pdf->AddPage();
        $x = $_x;
        $y = $_y;
    }
}

$pdf->Output('F', 'thumbnails.pdf');