How do I create an idled timeout in PHP using sessions?

I was wondering how to go about inserting a time out function on my website using sessions in PHP.  Basically, I am looking for a timeout code that logs a user out and sends them back to the login (index.php) page after the user is idle for 5 mins.

Would I first need to create a 'timeout' field in my SQL database?  If so, what information would I need to do that; or what values, etc. do I need to set that field equal to?

Here is my index.php, which is a login page for my website.  If someone can take a look over the code, and provide me with any instructions or directions I need to take in order to get a timeout session working, I would appreciate it!!!

index.php
=================================
<?php
require_once('db.php');
include('functions.php');

if(isset($_POST['Login']))
{
      if($_POST['username']!='' && $_POST['password']!='')
      {
            //Use the input username and password and check against 'users' table
            $query = mysql_query('SELECT ID, Username, Active, Level_access FROM users WHERE Username = "'.mysql_real_escape_string($_POST['username']).'" AND Password = "'.mysql_real_escape_string(md5($_POST['password'])).'"');
                  
            if(mysql_num_rows($query) == 1)
            {
                  $row = mysql_fetch_assoc($query);
                  if($row['Active'] == 1)
                  {
                  session_start();
                  $_SESSION['user_id'] = $row['ID'];
                  $_SESSION['logged_in'] = TRUE;
                              
                                 switch($row['Level_access'])
                  {
                      case 1:
                  {
                           header("Location: control_panel.php");
                            break;
                  }
                      case 2:
                  {
                            header("Location: members.php");
                            break;
                  }
                  }
            }
                  else {
                  $error = 'Your user account was not activated. Please open the email that was sent and click on the activation link.';
            }
      }
            else {            
            header("Location: login_fail.php");            
            }
      }
      else {
      $error = 'Please enter both your username and password to access your account';
      }
}
?>

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