A tutorial from phpmagazine.net which show you how to create an online visitor display using PHP and Ajax.

“There is many techniques to track online visitors with PHP, in this example I used the most simple one. The script check the sessions online and then consider the recent ones in an interval of about 3 minutes. This gives more realistic information, because visitors idle for a long period are considered not online, and in the same time we don’t have to create complex script and store informations in database”

Link: AJAX Magazine: AJAX Real Time Online Visitors Tutorial