http Last-Modified dont' work as expect

Question

Response:

Cache-Control:no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Connection:keep-alive
Content-Type:image/jpeg
Date:Tue, 23 Jul 2013 13:56:05 GMT
Etag:"76a5140090233de1521969b60c7636c1"
Expires:Thu, 19 Nov 1981 08:52:00 GMT
Last-Modified:Mon ,22 Jul 2013 10:16:06 GMT
Pragma:no-cache
Server:nginx/1.2.8
Set-Cookie:lang=en; expires=Thu, 22-Aug-2013 13:56:05 GMT; path=/
Set-Cookie:money=121; expires=Thu, 22-Aug-2013 13:56:05 GMT; path=/
Set-Cookie:UID=6; expires=Thu, 22-Aug-2013 13:56:05 GMT; path=/
Transfer-Encoding:chunked
X-Powered-By:PHP/5.4.13

And after Response the Request is:

Accept:text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Encoding:gzip,deflate,sdch
Accept-Language:zh-CN,zh;q=0.8
Cache-Control:no-cache
Connection:keep-alive
Cookie:sid=8cif822kqg6qo4vags10pbm697; PHPSESSID=su382iqj9pdgbl32gqd1fqff03;                     
Host:localhost
Pragma:no-cache
User-Agent:Mozilla/5.0 (Macintosh; Intel Mac OS X 10_8_3) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/27.0.1453.116 Safari/537.36
Response Headersview source

As you see,the request don't send If-Modified-Since header as expect.

Am I miss something else?My Browser don't disable cache.

Any one can help me?Thank you very much.

Answer 1

After diff nginx 304 header，I've noticed the reason.

The reason is start_session() add no cache header.After remove Prgama and Cache-Control header.Everything turn right.