see this line:

$etag = substr(sha1($secret . sha1($_SERVER["REMOTE_ADDR"]) . sha1($_SERVER["HTTP_USER_AGENT"])), 0, 18);

As we must track the user for the first document page( eg: http://lucb1e.com/rp/cookielesscookies/ in the example), bus this document request doesn't contain a ETAG header. Which means if you use a random string instead of user-agent and ip here, the user can't not be tracked here.
how to fix this?

Nothing really wrong with your code, just an FYI. A developer and I are both using Chrome on the same network. The changes I make are visible on his Chrome and the changes he makes are visible on my Chrome. This doesn't happen on Firefox nor Safari. Just thought this might be worth the mention. I'd be interested in knowing why this happens.

When we visit the tracker.jpg we can get it's Etag.But when we visit the website we can't get Etag,and everytime it will generate a new Etag. The “cookie” can work just because $etag = substr(sha1($secret . sha1($_SERVER["REMOTE_ADDR"]) . sha1($_SERVER["HTTP_USER_AGENT"])), 0, 18); It depend on IP and UA,everytime it genrate the same Etag.When IP or UA changed it will not work,and if the method of generate Etag is random it also can't work.

ETag support is not available in the old IE-browsers.

http://en.wikipedia.org/wiki/HTTP_ETag#Tracking_using_ETags

• I visited your page: http://lucb1e.com/rp/cookielesscookies/
• Now after your post... i refresh the page
• Page Visit Count increases to two
• and now i keep refreshing the page for a number of times...
  • the Page Visit Count increases with every refresh BUT
  • sometimes the count is reset to 2
  • on further refresh actions, the count again starts with two... goes up until some point and then again drops to 2
    • this keeps on happening, but there's no pattern when the count is reset to 2.

我看etag的生成方式，有可能同一局域网的多台电脑，session公用的情况，这个怎么解决呢

Hi,I have a question about the code here:https://github.com/lucb1e/cookielesscookies/blob/master/index.php#L55-L59.

I want to know why you need to read picture file when the eTag was matched (index.php line 59)？in my opinion,the data of picture will be send to client again.