php post 提交json包，返回false ，怎么回事？

php  post  提交json包，返回false  ，怎么回事？   /**
     * PHP发送Json对象数据
     *
     * @param $url 请求url
     * @param $data_string发送的json字符串
     * @return array
     */function http_post_data($url, $data_string) {        $ch = curl_init($url);                                                                      
        curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");                                                                     
        curl_setopt($ch, CURLOPT_POSTFIELDS, $data_string);                                                                  
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);                                                                      
        curl_setopt($ch, CURLOPT_HTTPHEADER, array(                                                                          
            'Content-Type: application/json',                                                                                
            'Content-Length: ' . strlen($data_string))                                                                       
        );                                                                                                                   
        $result = curl_exec($ch);        return $result;
    }