how to set a session time for specific page

When you go to this page; http://www.ctuchicago.com/registration/login.php

you will see the form that I made. I want to create a session if username and password is true than I wanna set a 20 minutes expire time for it. How can I do it ?

my check.php

$user = $_POST['user'];
$pass = $_POST['pass'];

$ruser = "a";
$rpass = "b";

if ($user == $ruser) {
 $logn = "True"; 
} else {
 $logn = "False";  
}

if ($pass == $rpass) {
 $logs = "True"; 
} else {
 $logs = "False";  
}


 if ($logn == "False" && $logs == "False") {
 echo '<script type="text/javascript">alert("The Username and The Password are both wrong ! Please check your information and try again.");history.go(-1);</script>';
 } elseif ($logn == "True" && $logs == "False") {
 echo '<script type="text/javascript">alert("The Username is True but something is wrong with the password you entered. Please check your information and try again.");history.go(-1);</script>';
 } elseif ($logn == "False" && $logs == "True") {
 echo '<script type=开发者_JAVA技巧"text/javascript">alert("The Password is True but something is wrong with the password you entered. Please check your information and try again.");history.go(-1);</script>';
 } else {
 $_SESSION['valid'] = "1";
 echo '<script type="text/javascript">window.location = "http://www.ctuchicago.com/registration"</script>';
 }

my index.php (this page I want to be login required.

    if ($_SESSION['valid']) {
 echo '<script type="text/javascript">window.location = "http://www.ctuchicago.com/registration/login.php"</script>';
} else { Code }

Thank You

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According to http://prajapatinilesh.wordpress.com/2009/01/14/manually-set-php-session-timeout-php-session/

ini_set(’session.gc_maxlifetime’, 20*60);

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Have you considered using a [client side] cookie that expires in 20 minutes ? then whenever the page is (re)loaded, check for the cookie, invalidate the session if the cookie is invalid/expired.

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See How do I expire a session after 30 minutes

EDIT

Basically what you are doing is creating your own session timeout function. When you hit the page you look for a variable you set in $_SESSION lets call it $_SESSION['started']. If you don't find that variable then you create it and give the value of the current timestamp. What you do then is every time you load a new page, you compare the $_SESSION['started'] variable with the time now and if it exceeds 20 minutes then you expire log the user out or do whatever you need to do.