CodeIgniter画像ファイルをアップロードできません

画像なしでデータを保存する場合

public function create() {
    //check if user is logged in
    if (!$this->session->userdata('logged_in')) {
      redirect('users/login');
    }

    $this->form_validation->set_rules('mjestoPolaska', 'Mjesto Polaska', 'required');
    $this->form_validation->set_rules('mjestoOdredista', 'Mjesto Odredista', 'required');
    $this->form_validation->set_rules('datumPolaska', 'Datum Polaska', 'required');
    $this->form_validation->set_rules('datumPovratka', 'Datum Povratka', 'required');
    $this->form_validation->set_rules('cijena', 'Cijena', 'required');
    $this->form_validation->set_rules('brojMjesta', 'Broj mjesta', 'required');
    $this->form_validation->set_rules('opis', 'Opis', 'required');


    $data['title'] = 'Create Posts';
    $data['categories'] = $this->Posts_model->get_categories();

    if ($this->form_validation->run() === FALSE) {

      $this->session->set_flashdata("error", validation_errors());
      $this->load->view('templates/header');
      $this->load->view('posts/create', $data);
      $this->load->view('templates/footer');
    } else {
      $post_image='';
      if (!empty($_FILES['userfile']['name'])) {
        $path = 'assets/images/posts/';
        $post_image = $this->ImageUpload($path, 'userfile', '');
        if (!is_array($post_image)) {

          $this->session->set_flashdata('error', $post_image);
          redirect(site_url() . 'posts/create');
        }
        $post_image = $post_image['file_name'];
      }


      $this->Posts_model->create_post($post_image);
      $this->session->set_flashdata('post_creted', 'You post has been created');
      redirect('posts');
    }
  }