HttpResponse response 回傳字串content_length = -1 問題

Question

php環境： THINKPHP5

C#程式碼： 

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

#request.KeepAlive = false;

#request.Method = "GET";

response = (HttpWebResponse) request.GetResponse();

php 為exit($content);

#如果$content長度超過8000，則response的content_length = -1,小於8000則為真實長度，請教高手這是什麼回事？