public function post($body) {  
  $this->setLine('POST');

  // 重新设置content-type
  $this->setHeader('Content-Type:application/json; charset=UTF-8');
  
  // 跳过setBody方法
  // $this->setBody($body);
  $this->body[]=$body;

  // 计算content-length
  $this->setHeader('Content-length: ' . strlen($this->body[0]));

  $this->request();

  return $this->response;
 }

<?php

require "http.class.php";


$http=new Http('http://www.cnblogs.com/mvc/PostComment/Add.aspx');
//设置头信息
$http->setHeader('Accept-Language:zh-CN,zh;q=0.8,en-US;q=0.5,en;q=0.3');
$http->setHeader('Accept-Language:zh-CN,zh;q=0.8,en-US;q=0.5,en;q=0.3');
$http->setHeader('Accept-Encoding:gzip, deflate');
$http->setHeader('X-Requested-With:XMLHttpRequest');
$http->setHeader('Referer:http://www.cnblogs.com/DeanChopper/p/4688667.html');
$http->setHeader('Cookie:_ga=GA1.2.1359064105.1438444082; __gads=ID=e0c32fd6db6e2a6d:T=1438443900:S=ALNI_Mb6AAflcBD6gcdHgeE3IqVDJYnnjA; .CNBlogsCookie=C8013C91E54C151DEDA30E2C1E842982338C9054A8BB8639AC2DAB7578445BF1DF5BC49D39D8BE5FDAC33541CE4E4FA386CFD3F946EA1D79D1E34809A4CCBD7488A15641AEF685A6258CF3F03597BCAF50049F8C95A310076677598990FB2E4FB1E9671A; _5t_trace_sid=84a9ed0b086c2c127551cf911bec7b1d; _5t_trace_tms=1; _gat=1');
$http->setHeader('Pragma:no-cache');
$http->setHeader('Cache-Control:no-cache');

//设置请求体信息
$msg='{"blogApp":"DeanChopper","postId":4688667,"body":"测试内容","parentCommentId":0}';

//发送post请求
$http->post($msg);

echo 'OK';