สอบถามเกี่ยวกับ function extract($_POST); ครับ codeigniter ครับ

public function index()
{
 $user_id = $this->input->post('user_id');
 $user_name = $this->input->post('user_name');
 $user_surname = $this->input->post('user_surname');
 $submit = $this->input->post('submit');
 $param['user_id']=$user_id;
 $param['user_name']=$user_name;
 $param['user_surname']=$user_surname;
 if(isset($submit))
 {
  $this->mod_user->update($param);
 }
 $this->load->view('register/index');
}

public function index()
{
 $param['user_id']=$this->input->post('user_id');
 $param['user_name']=$this->input->post('user_name');
 $param['user_surname']=$this->input->post('user_surname');
 if(isset($this->input->post('submit')))
 {
  $this->mod_user->update($param);
 }
 $this->load->view('register/index');
}

<form>
<input type=text name="ro[user_id]">
<input type=text name="ro[user_name]">
<input type=text name="ro[user_surname]">
<input type=submit name=submit value=submit>

if(isset($this->input->post('submit'))){
  $this->mod_user->update($this->input->post('ro'));
}