php修改json中指定的值

  <?php

header("content-type:text/html;charset=utf-8");

$postid=$_REQUEST['postid'];

$url ="";

    $ch = curl_init();

    //设置选项，包括URL

    curl_setopt($ch, CURLOPT_URL, $url);

    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);

    curl_setopt($ch, CURLOPT_HEADER, 0);

    //执行并获取HTML文档内容

    $output = curl_exec($ch);

$arr = json_decode($output,true);

    $getcom=$arr['auto'][0]['comCode'];

include_once("kuaidi_config.php");

$url ="";

    //设置选项，包括URL

    curl_setopt($ch, CURLOPT_URL, $url);

    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);

    curl_setopt($ch, CURLOPT_HEADER, 0);

    //执行并获取HTML文档内容

    $output = curl_exec($ch);

    //释放curl句柄

    curl_close($ch);

$obj=json_decode($output, true);

foreach($obj as $key=>$value){

$obj['com']="中通";

}
echo json_encode($obj);
?>