Session Timeout question

What is the best way to set a user timeout on a session?
Say if the user steps away from their computer for say 5 min,
and they come back to the page they were on, I want when the user  
clicks something
for the session to kick them off and redirect to login saying their  
session has timed out.
I have this code currently:


   /*Users timout*/
   function getIdle(){
   global $database;
   	if($this->isGuest()) {
	      if (!isset($_SESSION['timeout_idle'])) {
			    $_SESSION['timeout_idle'] = time() + GUEST_TIMEOUT;
			} else {
			    if ($_SESSION['timeout_idle'] < time()) {
			        //destroy session
			        return(true);
			    } else {
			        $_SESSION['timeout_idle'] = time() + GUEST_TIMEOUT;
			        return(false);
			    }
		  }
	  } else {
	      if (!isset($_SESSION['timeout_idle'])) {
			    $_SESSION['timeout_idle'] = time() + USER_TIMEOUT;
			} else {
			    if ($_SESSION['timeout_idle'] < time()) {
			        //destroy session
			        return(true);
			    } else {
			        $_SESSION['timeout_idle'] = time() + USER_TIMEOUT;
			        return(false);
			    }
		  }
	  }
    }

But it doesn't time anything out.. I could walk away for an hour and  
come back and go to a link.
TIA,


Karl DeSaulniers
Design Drumm
http://designdrumm.com