function auto_login($userID) {
    if (!is_user_logged_in()) {
        
        $user = get_userdata($userID);
        $user_id = $user->ID;
        $user_login = $user->user_login;
        
        //login
        wp_set_current_user($user_id, $user_login);
        wp_set_auth_cookie($user_id);
        do_action('wp_login', $user_login);
    }
} 
add_action('user_register', 'auto_login');

function auto_login($userID) {
    if (!is_user_logged_in()) {
        //determine WordPress user account to impersonate        
        
        //get user's ID
        $user = get_userdata($userID);
        $user_id = $user->ID;
        $user_login = $user->user_login;
        
        //login
        wp_set_current_user($user_id, $user_login);
        wp_set_auth_cookie($user_id);
        do_action('wp_login', $user_login);
    }
} 
add_action('user_register', 'auto_login');

Code snippet that allows the user to be automatically logged in on account creation, this works with s2Member since they allow the user to create their own password.

WordPress AutoLogin

login, wordpress

PHP