Trying to build a cute little php site statistics script, now I'm stuck with getting the average visiting time on site.

How to do that? I'm using JS badge.

HTTP is stateless, if that is the case how the server knows when the visitor navigates away? JS onunload?

If you're going to do it with JavaScript then yep, onunload or onbeforeunload would definitely be one way to do it. Just keep in mind that not all of your users will have JavaScript enabled.

I'd use that in conjunction with sessions with the assumption that many of the people who visit your site will be navigating to another page on your site.

If you're tracking this in a database, for any open table rows you have that the page view time is ridiculous, you could just place a marker on it, and when you run your stats either throw it out, or assume the time spent on that page is the same as your calculated average for that page. Just a thought

Thanks for the heads-up. Now I wonder how most of the site statistics / logs analyzers do that.

With respect to time spent viewing a page -- not very well.

You said it, HTTP is a stateless protocol. I have no clue if a customer of mine sat and read a web page for six hours, or loaded it and left the room for a smoke.

With respect to marketing analysis, I (personally, not saying it may not be important for you or others) have no use for that data. I care more about what pages are viewed, where the user was previously at, and where the user went to. I just want to know correlations; primarily so I can find out what I need to do better to make a sale.

Google Analytics is very good in this respect -- maybe they've got a page up there (hopefully that doesn't involve pigeons) explaining how their system works.