Help with Last Activity Time

[Pro2000]

Peace, I wrote a PHP File Contains a login form, If the user logs in a field will be inserted into the MySQL DataBase, So I want to make an automatic logout after 20 minutes idle.
I wrote:

<?php
session_start();
require("Connecting_Config.php");
$unm=$_SESSION['user_name'];
$current_date=date("i"); //Only Minutes
$gt_lastactivity="select * from logs where Name='$unm'";
$do_gt_lastactivity=mysql_query($gt_lastactivity,$connect);
$arr=mysql_fetch_array($do_gt_lastactivity);
$last_activity_date=$arr['Last_Activity'];
$yyy=$current_date-$last_activity;
if ($yyy>20)
{
          Logout();
}
?>

I tried that and I encountered an error which does a logout to the user if he starts another hour.. Ex: Last_Activity= 59th Minute If he does a refresh in the 1st minute in the next hour it will logout, Who knows how to do that?

[fatihpiristine]

try cookies. that might help you. this example from O'Reilly's Learning PHP5 (free: chapter 8)

// The cookie expires one hour from now
setcookie('short-userid','ralph',time( ) + 60*60);
// The cookie expires one day from now
setcookie('longer-userid','ralph',time( ) + 60*60*24);
// The cookie expires at noon on October 1, 2006
setcookie('much-longer-userid','ralph',mktime(12,0,0,10,1,2006));

[Pro2000]

try cookies. that might help you.

Sorry but I know this... I created a menu shows the available users, It gets the names from the mysql database, I need to make it when the user expires 20 minutes it will logout anddelete the user name from the available users.

[fatihpiristine]

then you will check it with cookies again and if the given time is up, redirect the page the username from availables list.