钉钉服务器企业发送消息40035错误处理

最近打算试用一下 钉钉的api接口，写了一个简单的php发送接口，就是提示  {"errcode":40035,"errmsg":"不合法的参数"}

这个只能在服务端看吗

<?php


$url = "https://oapi.dingtalk.com/message/send_to_conversation?access_token=bd3cd13e8____8db8c0171db_____";
$post_data = '{"touser":"manager___","agentid":"12857___,"msgtype":"text","text":{"content":"hello world"}}';
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_HTTPHEADER,  array('Content-Type: application/json; charset=utf-8'));


curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $post_data);


$output = curl_exec($ch);


curl_close($ch);
print_r($output);






?>