It seems like a two-step process to get this,
$u = $this->Auth->user();
$uid = $u['User']['id'];
Isn't there a variable set somewhere once a user is logged in?
You can use $uid = $this->Auth->user('id');
Check the api documentation: https://api.cakephp.org/1.3/class-AuthComponent.html#_user
For everybody using the newer Cakephp v4.x: https://api.cakephp.org/4.4/class-Cake.Controller.Component.AuthComponent.html#user()