<?php
// Fill these out with the values you got from Google
$googleClientID = '';
$googleClientSecret = '';

// This is the URL we'll send the user to first to get their authorization
$authorizationEndpoint = 'https://accounts.google.com/o/oauth2/v2/auth';

// This is Google's OpenID Connect token endpoint
$tokenEndpoint = 'https://www.googleapis.com/oauth2/v4/token';

// The URL for this script, used as the redirect URL
// If PHP isn't setting these right you can put the full URL here manually
$protocol = isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] === 'on' ? 'https' : 'http';
$redirectURL = $protocol . '://' . $_SERVER['HTTP_HOST'] . $_SERVER['PHP_SELF'];

// Start a session so we have a place to store things between redirects
session_start();



// Start the login process by sending the user
// to Google's authorization page
if(isset($_GET['action']) && $_GET['action'] == 'login') {
  unset($_SESSION['user_id']);

  // Generate a random hash and store in the session
  $_SESSION['state'] = bin2hex(random_bytes(16));

  $params = array(
    'response_type' => 'code',
    'client_id' => $googleClientID,
    'redirect_uri' => $redirectURL,
    'scope' => 'openid email',
    'state' => $_SESSION['state']
  );

  // Redirect the user to Google's authorization page
  header('Location: ' . $authorizationEndpoint . '?' . http_build_query($params));
  die();
}

if(isset($_GET['action']) && $_GET['action'] == 'logout') {
  unset($_SESSION['user_id']);
  header('Location: '.$redirectURL);
  die();
}

// When Google redirects the user back here, there will be a "code" and "state"
// parameter in the query string
if(isset($_GET['code'])) {
  // Verify the state matches our stored state
  if(!isset($_GET['state']) || $_SESSION['state'] != $_GET['state']) {
    header('Location: ' . $redirectURL . '?error=invalid_state');
    die();
  }

  // Exchange the auth code for a token
  $ch = curl_init($tokenEndpoint);
  curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
  curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query([
    'grant_type' => 'authorization_code',
    'client_id' => $googleClientID,
    'client_secret' => $googleClientSecret,
    'redirect_uri' => $redirectURL,
    'code' => $_GET['code']
  ]));
  $response = curl_exec($ch);
  $data = json_decode($response, true);

  // Note: You'd probably want to use a real JWT library
  // but this will do in a pinch. This is only safe to do
  // because the ID token came from the https connection
  // from Google rather than an untrusted browser redirect

  // Split the JWT string into three parts
  $jwt = explode('.', $data['id_token']);

  // Extract the middle part, base64 decode it, then json_decode it
  $userinfo = json_decode(base64_decode($jwt[1]), true);

  $_SESSION['user_id'] = $userinfo['sub'];
  $_SESSION['email'] = $userinfo['email'];

  // While we're at it, let's store the access token and id token
  // so we can use them later
  $_SESSION['access_token'] = $data['access_token'];
  $_SESSION['id_token'] = $data['id_token'];
  $_SESSION['userinfo'] = $userinfo;

  header('Location: ' . $redirectURL);
  die();
}



// If there is a user ID in the session
// the user is already logged in
if(!isset($_GET['action'])) {
  if(!empty($_SESSION['user_id'])) {
    echo '<h3>Logged In</h3>';
    echo '<p>User ID: '.$_SESSION['user_id'].'</p>';
    echo '<p>Email: '.$_SESSION['email'].'</p>';
    echo '<p><a href="?action=logout">Log Out</a></p>';

    echo '<h3>ID Token</h3>';
    echo '<pre>';
    print_r($_SESSION['userinfo']);
    echo '</pre>';

    echo '<h3>User Info</h3>';
    echo '<pre>';
    $ch = curl_init('https://www.googleapis.com/oauth2/v3/userinfo');
    curl_setopt($ch, CURLOPT_HTTPHEADER, [
      'Authorization: Bearer '.$_SESSION['access_token']
    ]);
    curl_exec($ch);
    echo '</pre>';

  } else {
    echo '<h3>Not logged in</h3>';
    echo '<p><a href="?action=login">Log In</a></p>';
  }
  die();
}

