问Google App Engine，AJAX返回PHP文件的内容字符串，而不是JSON

EN

application: app353
version: 1
runtime: php
api_version: 1
threadsafe: yes

handlers:
- url: /favicon\.ico
static_files: favicon.ico
upload: favicon\.ico

- url: /css
static_dir: css

- url: /js
static_dir: js

- url: .*
script: index.php

- url: /php/genjson
script: php/genjsonphp.php

<?php

?> 
<script src="./js/jquery.js"></script>
<script type="text/javascript" language="javascript" src="./js/jquery.dataTables.js">     
</script>
<link rel="stylesheet" type="text/css" media="all" href="./css/jquery.dataTables.css" /> 


<script>
$(document).ready(function() 
{    
$('#example').dataTable( 
{  
 "ajax": './php/genjsonphp.php' 
 } );
} );
</script>


 <table class="display" id="example">
 <thead>
 <th>Dimensions</th>

 </thead>
 <tbody></tbody>
 </table> 

css/*.css
js/*.js
php/genjsonphp.php
app.yaml
index.php

header('Content-Type: application/json');
var_dump(json_decode($strjSon));
header('Content-Type: application/json');
echo $strjSon;