Fillable PDF Form with PHP using PDFtk error - php

Question

I am trying to fill out pdf files using a github project that i found on [https://github.com/mrdigitalau/PHP-PDFTK-Tutorial]
Basically there is 2 page, one named: GeneratedPDF that hold code:
<?php
namespace Classes;
if(!defined('ACCESSCHECK')) {
die('Direct access not permitted');
}
use mikehaertl\pdftk\Pdf;
class GeneratePDF {
public function generate($data)
{
try {
$filename = 'pdf_' . rand(2000,1200000) . '.pdf';
$pdf = new Pdf('./test.pdf');
$pdf->fillForm($data)
->flatten()
->saveAs( './completed/' . $filename);
//->send( $filename . '.pdf');
return $filename;
}
catch(Exception $e)
{
return $e->getMessage();
}
}
}
And another page name generate.php that holds this code:
<?php
if ($_SERVER['REQUEST_METHOD'] != 'POST') {
exit;
}
define('ACCESSCHECK', TRUE);
require_once 'vendor/autoload.php';
use Classes\GeneratePDF;
$data = [
'name_field' => $_POST['fname'] .' ' . $_POST['lname'],
'email_field' => $_POST['email'],
'phone_field' => $_POST['phone'],
'enquiry_field' => $_POST['enquiry']
];
$pdf = new GeneratePdf;
$response = $pdf->generate($data);
header('Location: thanks.php?fname=' . $_POST['fname'] . '&link=' . $response);
The problem is every time i try to fill out a pdf I get this error:
Fatal error: Uncaught Error: Class 'Classes\GeneratePDF' not found in C:\xampp\htdocs\generate.php:23 Stack trace: #0 {main} thrown in C:\xampp\htdocs\generate.php on line 23
Any idea the problem here? Thanks in advance