即使url返回{“成功”：true}，Extjs表单成功函数也不会执行

Question

1. Php Code

    <?php
      Class semester extends CI_Controller
      {
        public function index()
         {
           echo '{"success":true}';
         }
      }
    ?>
   

2. ExtJS代码

   var form=this.up('form').getForm();
   form.url = 'http://localhost/anju/index.php/semester';
   form.method = 'POST'; 
   form.submit({
         success: function (form, action) {
            alert("Success: " );
        },
        failure: function (form, action) {
            alert("Failure: " );
        }
      });
   

到目前为止，我无法取得成功的消息:(有人可以告诉我它失败的原因吗？