$post_xml='<xml>   <appid>'.$post['appid'].'</appid>   <mch_id>'.$post['mch_id'].'</mch_id>   <nonce_str>'.$post['nonce_str'].'</nonce_str>   <receiver>{ "type":"PERSONAL_OPENID", "RELATION_TYPE":"DISTRIBUTOR", "ACCOUNT":'.$openid.'} </receiver>   <sign>'.$sign.'</sign>   </xml>'; 这个表达错哪了，一运行就提示接收方格式错误