I've done a forum site before and the 'online visitors' wasn't a problem. Thing is, having had this, I liked knowing this info and would very much like to have it available on my other sites - even if it is only visible to me.
I have one on all my sites and just have it as a small icon in the bottom corner. There's a section on there that gives you all the details of who's online right now.
You could also use your site host stats but they rarely tell you who's online though.
loads of free visitor tracking software around. We use statcounter (www.statcounter.com) and its completely free! Great for examining who is coming in from various referrals. Google analytics is another such tool.
If you want to be really posh try visitorville where you can see you visitors coming on in 3d via buses and cars. Pages are represented by buildings etc...brill.
WhosOn is quite good for that - shows the visitors IP, country, referred from, path through site so far etc - think you can leverage it to use as a chat tool also - so you could jump out on an unsuspecting visitor and demand to know why they haven't bought something!!
Does normal stats also (not the best but not the worst either)
Just to clarify, by 'how many people are online' I assume you want to know how many people are looking at your website at any given moment.
For a transaction based protocol like HTTP, that's really a bit meaningless... If you're doing some cool ajax then fine, people are 'online', and you know when they disconnect.
So the usual method to find out approx number of users 'online' is to count the visits in the last N seconds. So you can report that in the last 30 seconds, 9 people have requested pages, so you're gonna say they are 'online'.
Another way to look at it is to say that visitors 'sessions' expire after N seconds of inactivity.
You could put something together in PHP to that effect pretty quickly, either using php sessions, or say mysql.
Most hosting packages will come with stats packages. I use AWStats more often than not....gives you plenty of detailed information about whats going on on your site, sessions, hits, and much more.
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