mongoDB, ajax, php to only display new messages - php

Question

It might be a silly question, but I think it will be useful for all new mongoDB users and not just myself.
I am currently working on a live chat script as a way to learn about mongoDB. I am having trouble with only loading new messages into chat rather than loading all messages and replacing old ones. This is what I have.
My PHP:
try
{
$m = new Mongo(); // connect
$db = $m->selectDB("local");
}
catch ( MongoConnectionException $e )
{
echo '<p>Couldn\'t connect to mongodb, is the "mongo" process running?</p>';
exit();
}
$collection = $db->selectCollection("message");
echo "".$collection." selected";
$cursor = $collection->find();
// iterate cursor to display title of documents
foreach ($cursor as $document) {
echo $document["sender"].": ". $document["message"] . "<br>";
My JS:
function get_new(){
var messages = $.ajax({
type: "POST",
url: "ajax/receive.php",
async: false
}).success(function(){
setTimeout(function(){get_new();}, 10000);
}).responseText;
$('div.messages').html(messages);
}
My messages collection:
"createdAt" => $now,
"sender" => $sender,
"message" => $message
I know that my JS would eventually start using .append(messages), but I really don't know what to do with my PHP. So how do I change my php to only find new messages (older than last set of messages)

Answer 1

Found the answer. Use sessions to store the time of last update. So The PHP code will look like this:
$now = date("Y-m-d H:i:s");
$last = $_SESSION["last_message_ts"];
$session_last = new MongoDate(strtotime($last));
$session_now = new MongoDate(strtotime($now));
$_SESSION["last_message_ts"]=$now;
try
{
$m = new Mongo(); // connect
$db = $m->selectDB("local");
}
catch ( MongoConnectionException $e )
{
echo '<p>Couldn\'t connect to mongodb, is the "mongo" process running?</p>';
exit();
}
$collection = $db->selectCollection("message");
$cursor = $collection->find(array("createdAt" => array('$gt' => $session_last, '$lte' => $session_now)));
// iterate cursor to display title of documents
foreach ($cursor as $document) {
echo $document["sender"].": ". $document["message"];
}