sso.module

<?php

/**
 * Implements hook_config_info().
 */
function sso_config_info() {
  return [
    'sso.settings' => [
      'label' => t('Single Sign-On Settings'),
      'group' => t('SSO'),
      'description' => t('Stores configuration settings for the Single Sign-On (SSO) module.'),
    ],
  ];
}

/**
 * Implements hook_init().
 * Check the sessions table for an existing valid session on page load.
 */
function sso_init() {
  if (user_is_logged_in()) {
    return; // Skip if the user is already logged in.
  }

  // Retrieve the session ID from the Backdrop native cookie.
  $cookie_name = session_name();
  $sid = $_COOKIE[$cookie_name] ?? NULL;

  if ($sid) {
    // Look up the session in the sessions table.
    $session = db_select('sessions', 's')
      ->fields('s', ['uid', 'timestamp'])
      ->condition('sid', $sid)
      ->condition('timestamp', REQUEST_TIME - ini_get('session.gc_maxlifetime'), '>')
      ->execute()
      ->fetchAssoc();

    if ($session && !empty($session['uid'])) {
      $account = user_load($session['uid']); // Load the user account.
      if ($account) {
        user_login_finalize($account); // Log in the user.
      }
    }
  }
}