I am having table to track user online status by setting \'is_online\' field to 1. Now User get logged in the site and shut down his/her system or power down at that time he
you can't. HTTP closes the connection once the page has been delivered. On the server side alone all you could do is checking if a user has requested a page within a certain amount of time.
JavaScript might be your friend. You could use it to create a ping to a php script which then updates the last request timestamp. Once there is no more pinging, the user is gone and you'd know.
If you have trouble with javaScript, a good point to start would be here: http://www.w3schools.com/JS/
You might want to focus on the AJAX and HTTP request stuff.
cu Roman