HOOK SET-UP PHP

I have this in my "hooks.php" file

$hook['post_controller_constructor'] = array(
  'class'    => 'My_hook',
  'function' => 'post_controller_constructor',
  'filename' => 'my_hook.php',
  'filepath' => 'hooks',
  'params'   => array()
);

this is in my file "my_hook.php" `

class My_hook {
  public function post_controller_constructor() {
    $CI =& get_instance();
    $matrix = $CI->user->privileges($_SESSION['user_id']);
    $CI->load->vars(array('matrix' => $matrix));
  }
}

So what and where do I put in my Controllers and where to get the array that i hope is being produced?