Create Authentication Cookie Outside of WordPress

Creating the WP cookies is painful and difficult.

A better way would be to create a WordPress plugin that recognizes the authentication from your other site’s cookie, if you have one.

This is actually quite easy to do, really. The authenticate filter makes it simple:

add_filter('authenticate','custom_login_check', 100);
function custom_login_check($user) {
// check if user is already logged in and just return if he is
if ( is_a($user, 'WP_User') ) { return $user; } 

// do whatever you have to do to check for authentication here
// in the end, you need to determine which WP user is logged in
// then return a WP_user object.

// example: say you figured out that it was user number 123
$user = new WP_User(123);

// always return the $user from the function argument
// even if you didn't change it
return $user;
}

That’s all there is to it, really. How you implement the authentication check is up to you.

Sure, my own Simple Facebook Connect and Simple Twitter Connect plugins use this same basic method to let you login using Facebook or Twitter credentials.