问Codeigniter - Ajax不工作无返回值

EN

echo '<script>$( "#start").click(function() {
    $.ajax({
      type: "POST",
      url: "'. base_url() .'StartTest/session",
      data: "id=' .$row->id .'",
      success: function (result) {
        alert(result);
        if(result == "' .$row->id .'"){
          window.location.replace("'. base_url() .'StartTest/index/0");
        }
      }
    });  
  });
</script> ';

public function session(){
    if($this->input->post('id')){
        $s_v = array(
            'testid' => $this->input->post('id')
            );
       $this->session->set_userdata($s_v);
       echo $this->session->userdata('testid');
    }else{
        echo "asd";
    }
}
public function index(){
    // Get Test ID
    $id = $this->session->userdata('testid');
    ...
    $this->pagination->initialize($config);

    $page = ($this->uri->segment(4)) ? $this->uri->segment(4) : 0;
    $data['question'] = $this->TestModel->getAllQuestion($id, $page);
    $data['links'] = $this->pagination->create_links();
    $this->load->view('TestView',$data);
}