The idea is to create a wordpress user when a user signs up on your cakephp site.

1. We need wordpress password hash function to do this properly. You need to create a new component that would contain wp password hashing functions. The file that contains everything you need is at wp-include/class-phpass.php. Just turn this class into a cakephp component, so you can include it in your user controller.

2. In your user controller, include your recently created wp hash function component. And after the user is saved into your cakephp user table. You want to add it to your wp user table as well. And it would look something like this.

if($this->User->save($this->data))
{
//create blog user for this user
$wp_hash = new $this->Wphash;
$wp_hash->PasswordHash(8,1);
$blog_user['ID'] = $this->User->id;
$blog_user['user_login'] = $this->sanitize_user($email);
$blog_user['user_pass'] = $wp_hash->HashPassword($this->data['User']['password']);
$blog_user['user_email'] = trim($email);
$blog_user['user_url'] = 'http://';
$blog_user['user_nicename'] = $this->data['User']['firstname'].' '.substr($this->data['User']['lastname'],0,1);
$blog_user['display_name'] = $this->data['User']['firstname'].' '.substr($this->data['User']['lastname'],0,1);
$blog_user['user_registered'] = date(configure::read('dateformat'));
$blogdata['Blog_user'] = $blog_user;
$this->Blog_user->save($blogdata);
//create user meta
$bid = $this->Blog_user->id;
$user_meta['user_id'] = $bid;
$user_meta['meta_key'] = 'first_name';
$user_meta['meta_value'] = $this->data['User']['firstname'];
$blogdata['BlogUserMeta']= $user_meta;
$this->BlogUserMeta->save($blogdata);
$user_meta['user_id'] = $bid;
$user_meta['meta_key'] = 'last_name';
$user_meta['meta_value'] = $this->data['User']['lastname'];
$blogdata['BlogUserMeta']= $user_meta;
$this->BlogUserMeta->save($blogdata);
$user_meta['user_id'] = $bid;
$user_meta['meta_key'] = 'nickname';
$user_meta['meta_value'] = $email;
$blogdata['BlogUserMeta']= $user_meta;
$this->BlogUserMeta->save($blogdata);
$user_meta['user_id'] = $bid;
$user_meta['meta_key'] = 'rich_editing';
$user_meta['meta_value'] = 'true';
$blogdata['BlogUserMeta']= $user_meta;
$this->BlogUserMeta->save($blogdata);
$user_meta['user_id'] = $bid;
$user_meta['meta_key'] = 'comment_shortcuts';
$user_meta['meta_value'] = 'false';
$blogdata['BlogUserMeta']= $user_meta;
$this->BlogUserMeta->save($blogdata);
$user_meta['user_id'] = $bid;
$user_meta['meta_key'] = 'admin_color';
$user_meta['meta_value'] = 'fresh';
$blogdata['BlogUserMeta']= $user_meta;
$this->BlogUserMeta->save($blogdata);
$user_meta['user_id'] = $bid;
$user_meta['meta_key'] = 'blog_capabilities';
$user_meta['meta_value'] = 'a:1:{s:10:"subscriber";b:1;}';
$blogdata['BlogUserMeta']= $user_meta;
$this->BlogUserMeta->save($blogdata);

3. In your wordpress index.php file

//change the session name of your wordpress site to match that of your cakephp site.
//so session data can be shared.
session_name("mysite");

4. in your theme header.php(might not be the best place if you change your theme a lot)

//you read the user id
$user = $_SESSION['Auth']['User'];

if(!empty($user))
{
//you change the current user manually
wp_set_current_user($user['id']);
}

And there you have it.