cron - update database if session expires

Question

I have got a "users" table in mysql database, in one field I keep the info if the user is online.

Everything works fine apart of:

-if some user closes the browser, his online status doesn't change to offline.

I have been told that I could do that with cron, but I don't know anything about it.

I can save the user id in his session and make the php function to update the db.

But... I have no idea how to make the cron run that php (with the id from the session)

How easy is it to do?

Answer 1

It is fairly simple. This might be in your PHP file (call it update.php).

<?php
    /*connect to mysql as per normal*/
    // log users off (hopefully, you're using enum here, but for an example...)
    mysql_query( 'UPDATE USERS SET STATUS = \'LOGGED_OFF\' '.
               // whose last action was more than 14400 seconds ago (4 hours)
               'WHERE (CURRENT_TIMESTAMP() - TIMESTAMP(LAST_ACTION)) > 14400' ); 
?>

Then, open crontab:

crontab -e

and add this line to run the tast every 15 minutes (change the number 15 to another number if you like):

*/15 * * * * /path/to/update.php