define('SSO_CLIENT_ID', '');
define('SSO_CLIENT_SECRET', '');
define('SSO_METADATA', 'https://PROVIDER.COM/.well-known/openid-configuration');
define('SSO_REDIRECT_URI', URL_PUBLIC . 'sso/callback');

public static function action_signin() {
  AuthUser::load();
  if (AuthUser::isLoggedIn()) {
    redirect('/admin');
    return;
  }

  $metadata = self::http(SSO_METADATA);

  $_SESSION['state'] = bin2hex(random_bytes(5));
  $_SESSION['code_verifier'] = bin2hex(random_bytes(50));
  $code_challenge = self::base64_urlencode(hash('sha256', $_SESSION['code_verifier'], true));

  $authorize_url = $metadata->authorization_endpoint.'?'.http_build_query([
    'response_type' => 'code',
    'client_id' => SSO_CLIENT_ID,
    'redirect_uri' => SSO_REDIRECT_URI,
    'state' => $_SESSION['state'],
    'scope' => 'openid email profile',
    'code_challenge' => $code_challenge,
    'code_challenge_method' => 'S256',
  ]);

  redirect($authorize_url);
}

public static function action_callback() {
  if(!isset($_SESSION['state']) || !isset($_GET['state']) || $_SESSION['state'] != $_GET['state']) {
    die('Authorization server returned an invalid state parameter');
  }

  if(isset($_GET['error'])) {
    die('Authorization server returned an error: '.htmlspecialchars($_GET['error']));
  }

  $metadata = self::http(SSO_METADATA);

  $response = self::http($metadata->token_endpoint, [
    'grant_type' => 'authorization_code',
    'code' => $_GET['code'],
    'redirect_uri' => SSO_REDIRECT_URI,
    'client_id' => SSO_CLIENT_ID,
    'client_secret' => SSO_CLIENT_SECRET,
    'code_verifier' => $_SESSION['code_verifier'],
  ]);

  if (isset($response->error)) {
    die('Authorization server returned an error: '.htmlspecialchars($response->error));
  }

  $userinfo = self::http($metadata->userinfo_endpoint, [
    'access_token' => $response->access_token,
  ]);

  if (!isset($userinfo->sub)) {
    die('Error fetching access token');
  }

  $login = self::createOrLogin($userinfo);
  if (!$login) {
    die('Authentication request was not successful');
  }

  redirect('/dashboard');
}

#RFC 9457 - Problem Details for HTTP APIs