How to use attributes in controller or model?

[eluser]djuric[/eluser]
I have function in Model like this:

Code:

public function cats($post_id = null) {  

  $qveri = $this->db->query("SELECT * FROM posts WHERE id = $post_id");
  return $qveri->result_row();
}

This is in controller:

Code:

$data['cats'] = $this->clean_blue_model->cats($post_id);

My question would be CAN I pass attributes to functions inside controller, like I did above, I'm getting error like variable $post_id not defined

What's the right way to pass variable to function like this?