index.php

<?php
session_start();
// Include vendor autoload
include 'vendor/autoload.php';

// Your website name, or URL. It will be displayed in Google Authenticator
define("WEBSITE_URL", 'exampleWebsite');
// The username of the user
$username = "username";

$ht = new HOTPass();

// Generate a secret key for the user. You'll have to store it in your database
// For this example we generate only a secret key. The secret key will be the same when you will refresh this page because we want to verify that the current code given is the same as the one on your phone
echo "<h2>The secret key</h2>";
if (empty($_SESSION['secretKey'])) {
	$secretKey = $ht->userRandomKey();
	$_SESSION['secretKey'] = $secretKey;
}
else 
	$secretKey = $_SESSION['secretKey'];
echo $secretKey;

// Generate the QR-code. The user will have to scan it from the Google Authenticator app
echo "<h2>The QR-code</h2>";
echo "<img src=".$ht->getQRcodeURL($username, WEBSITE_URL, $secretKey)." />";

// You will have to call this method with the user's secret key to have the current right key
// Remember: a new key is generated every 30 seconds
echo "<h2>The current code</h2>";
$currentCode = $ht->generate($secretKey, 30);
echo $currentCode;
?>