HttpResponse response 返回字符串content_length = -1 问题

Question

php环境： THINKPHP5

C#代码： 

request = (HttpWebRequest) WebRequest.Creeate(uri);

request.KeepAlive = false;

request.Method = "GET";

response = (HttpWebResponse) request.GetResponse();

php 为exit($content);

如果$content长度超过8000，则response的content_length = -1,小于8000则为真实长度，请教高手这是什么回事？