WordPressis a great blogging engine. It’s flexible, scalable, and easy to tweak/configure to integrate into an existing PHP site. However, if you have an existing site with available user authentication and management capabilities, getting WordPress to accept those credentials (in a single sign-on fashion) can be a bit of a challenge.

Before we proceed, I should note that there are a number of available plugins that enable WordPress to integrate with some of the popular content management systems out there. Our requirement is a bit different however. We want to bypass WordPress’ authentication mechanism all together and have users login through the main portion of the site. In fact, in a well integrated site, the interface should make navigating between WordPress pages and the rest of the site seamless to the user. Our goal is to write a WordPress plug-in that will automatically authenticate a user who is already logged into the parent site (and, consequently, grant the user access to edit the blog’s content). All other users will have the rights of an unregistered visitor.

In my setup, the main site has role-based permissions and the WordPress setup only has one account for each role (i.e. admin, editor, user etc…). The plugin first checks the role of the user logged in to the main site and then simulates a WordPress login anytime the user navigates to the blog. You should be able to customize this method for your own needs.

function auto_login() {
    if (!is_user_logged_in()) {
        //determine WordPress user account to impersonate
        $user_login = 'guest'; 

        //get users password
        $user = new WP_User(0, $user_login);
        $user_pass = md5($user->user_pass); 

        //login, set cookies, and set current user
        wp_login($user_login, $user_pass, true);
        wp_setcookie($user_login, $user_pass, true);
        wp_set_current_user($user->ID, $user_login);
    }
}
add_action('init', 'auto_login');

Additional notes and caveats for the attentive reader

• There is a wp-include/pluggable.phpfile that defines all the functions that you can override and hook into. The WordPress API documentation is not very thorough so you may need to review the actual code.
• WordPress uses a double MD5 hash of the password to authenticate the user. In the database, the password is stored as a single hash. We need to hash that password again before passing it into the wp_login() function (and set the third parameter to indicate that the password is already hashed). Obviously hard coding the actual password would be a big no-no.

We did all this work to login but what about logging out? We have several options. First, we can call WordPress’ logout method which is wp_clearcookie()from the main site.  The drawback to this approach is that we need to include all the WordPress libraries into our main site for this to work (too much unnecessary overhead IMHO). The other approach is to not use cookies at all thus alleviating the need to logout. To do this we simply remove the call to wp_setcookie()in out plugin and override the auth_redirect()function to do nothing. This works because we impersonate the user on every page load and the only WordPress code that checks the cookie was in auth_redirect()until we got rid of it. Another side effect of this is that un-authenticated WordPress users will no longer be taken to the WordPress login page (but we didn’t want that anyway).

Update 6/4/08: There were a few changes to the WordPress API as of version 2.5 and some of the functions I used above became depreciated. The API documentation has also improved. A better way to implement the auto_login() function above is as follows.

function auto_login() {
    if (!is_user_logged_in()) {
        //determine WordPress user account to impersonate
        $user_login = 'guest';

       //get user's ID
        $user = get_userdatabylogin($user_login);
        $user_id = $user->ID;
  
        //login
        wp_set_current_user($user_id, $user_login);
        wp_set_auth_cookie($user_id);
        do_action('wp_login', $user_login);
    }
} 
add_action('init', 'auto_login');