Our task is to give browser enough information to identify content, which later can be used to decided if content has changed, and if not – say it to browser without sending content again. Bigger picture is explained here, I really recommend analysing it a little bit – it gives better understanding how headers work and what they mean.

As you saw in headers map, there are many possible headers to send, but we can focus on three of them:

1. Etag. Unique identified for output data (maybe hash)
2. Last-Modified. Date of last data modification
3. Expires. When to expire content

Constructing algorithm is pretty simple, it can be explained in pseudo-code:

get request headers
   is content expired
      send content, new headers
   else is content different (Etag)
      send content, new headers
   else
      304 header, no content

I’ve created outputCacheHeaders(…) function which (available here) outputs all required headers and returns true if you need to output content or false if not. All you need to do is pass modification time, live time and data (two last ones are not required). Live time says how long should cache be kept and data is used to generate Etag. Very simple!

After enabling correct headers sending, my websites started to feel much faster, because HTTP requests takes much less time. Also, Yahoo performance tips are really worth reading, really. There are many great tips and I have been using many of them for a long time. These tricks are much much more significant than 0.1 s. faster PHP rendering time.