为什么总是jQuery Ajax JSON请求失败？

I currently have a code snippet which I'd like to use to get Ajax Calls and return JSON data from my PHP file which I can use however I want in the jQuery side. My problem is that IF I change the data type to JSON, I always get error on the request, HOWEVER when I inspect it in Firebug I can see that the PHP file has just returned the JSON values fine!

我目前有一个代码片段，我想用它来获取Ajax调用并从我的PHP文件返回JSON数据，我可以在jQuery端使用它。我的问题是，如果我将数据类型更改为JSON，我总是会在请求中出错，但是当我在Firebug中检查它时，我可以看到PHP文件刚刚返回JSON值！

This is the HTML:

这是HTML：

<form id="formm" method="post">
    <input type="text" name="test" value="" id="test"/>
    <input type="submit" name="submit" value="Submit"/>
</form>

<div id="result"></div>

This is the JS:

这是JS：

$("#formm").submit(function(event) {

    /* Stop form from submitting normally */
    event.preventDefault();

    /* Clear result div*/
    $("#result").html('');

    /* Get some values from elements on the page: */
    var values = $(this).serialize();

    /* Send the data using post and put the results in a div */
    $.ajax({
        url: "functions.php",
        type: "post",
        dataType: "json",
        data: values,
        success: function(data) {
            alert(data);
        },
        error:function(){
            alert("failure");
            $("#result").html('There is error while submit');
        }
    });
});

PHP file:

PHP文件：

echo json_encode(array('returned_val' => $_POST['test']));

When I inspect with Firebug I get: returned_val "whatever I type in the textbox". Can anyone tell me what could be the problem?

当我用Firebug检查时，我得到：returned_val“无论我在文本框中输入什么”。谁能告诉我可能是什么问题？

UPDATE:

更新：

Response Headers:

响应标题：

Accept  application/json, text/javascript, */*; q=0.01
Accept-Encoding gzip, deflate
Accept-Language ***
Content-Length  5
Content-Type    application/json; charset=utf-8
Host    localhost
Referer http://localhost/test/
User-Agent  Mozilla/5.0 (Windows NT 6.1; WOW64; rv:24.0) Gecko/20100101 Firefox/24.0
X-Requested-With    XMLHttpRequest

When I put the errors in the console, I get this:

当我把错误放在控制台中时，我得到了这个：

The following error occured: parsererror SyntaxError: JSON.parse: unexpected character

发生以下错误：parsererror SyntaxError：JSON.parse：意外字符

1 个解决方案

#1

The JSON data is parsed in a strict manner; any malformed JSON is rejected and a parse error is thrown. As of jQuery 1.9, an empty response is also rejected; the server should return a response of null or {} instead

JSON数据以严格的方式解析;任何格式错误的JSON都会被拒绝，并抛出一个解析错误。从jQuery 1.9开始，空响应也被拒绝;服务器应该返回null或{}的响应

here is the documentation http://api.jquery.com/jQuery.ajax/

这里是文档http://api.jquery.com/jQuery.ajax/

so check your json response from php (whether the json data is well formed and non empty)

所以从php检查你的json响应（json数据是否格式正确且非空）

#1