接收到的json解析不了，必须用php解析,该如何解决

<br />
function sendPost($url,$postData){<br />
 $postdata = http_build_query($postData);<br />
 $options = array(<br />
 'http' => array(<br />
 'method' => 'POST',<br />
 'header' => 'Content-type:application/x-www-form-urlencoded',<br />
 'content' => $postdata,<br />
 'timeout' => 15 * 60 // 超时时间（单位:s）<br />
)<br />
 );<br />
 $context = stream_context_create($options);<br />
 $result = file_get_contents($url, false, $context);<br />
<br />
 return $result;<br />
 }<br />
<br />
<br />
$url = "http://chepiao.sinaapp.com/api.php?act=remain";<br />
$data = array(<br />
 "date"=>"20131220",<br />
 "startStation"=>"北京",<br />
 "arriveStation"=>"天津"<br />
);<br />
$result = sendPost($url, $data);<br />
 $obj = json_decode($result);<br />
$result就是返回来的数据，就是解析不了，用json_decode($result);解析成的是NULL