update database als session wordt verwijderd

function login(){
	$user = $_POST['gebruiker'];
	$pass = crypt($_POST['wachtwoord'], ' crypt even weggehaald;-) ');
	$sth = $GLOBALS['pdo']->prepare('SELECT * FROM gebruikers WHERE gebruiker = ? AND wachtwoord = ?');
	$sth->execute(array($user,$pass));
	$sth->setFetchMode(PDO::FETCH_OBJ);

	if ($sth->rowcount() == 1)
	{
		$row = $sth->fetch();
		$sth = $GLOBALS['pdo']->prepare('UPDATE gebruikers SET online = ? WHERE gebruiker = ?');
		$sth->execute(array(1, $user));
		$_SESSION['login'] = (array)$row;
		echo "success";
	}
	else {
		echo "ongeldige gebruikersnaam of wachtwoord";
	}
}

<?php 
	include_once('config.php');
?>

<div id="gebruikers">
	<div class="titel">Wie is er online?</div>
	<?php 
		whoIsOnline(); 
	?>
</div>
<div id="chat">
	<?php 
		getChat();
	?>
</div>

function whoIsOnline() {
	$sth = $GLOBALS['pdo']->prepare('SELECT gebruiker, avatar FROM gebruikers WHERE online = ?');
	$sth->execute(array(1));
	$sth->setFetchMode(PDO::FETCH_OBJ);

	while($row = $sth->fetch()) {
		echo '<div class="gebruiker">';
		echo $row->gebruiker;
		echo '</div>';
	}
}

function getChat() {
	$sth = $GLOBALS['pdo']->prepare('SELECT * FROM chats ORDER BY id desc LIMIT 20;');
	$sth->execute();
	$sth->setFetchMode(PDO::FETCH_OBJ);

	while($row = $sth->fetch()) {
		echo '<div class="message">';
		echo '<div class="avatar"></div>';
		echo '<p>'.$row->bericht.'</p>';
		echo '<div class="details">'.$row->auteur.', '.$row->tijd.'</div>';
		echo '<div class="clearfloat"></div>';
		echo '</div>';
	}
}