Getting the last inserted messages from ajax chat - php

Question

I'm building a chat application and having some issues. First of all I want the chat starts loading from database once the the user logs in.
function that gets the whole chat from db which is called by ajax
function getChatData(){
global $mysqli;
//echo $lastinseted= $mysqli->insert_id();
$sql = "SELECT `chat`.`message`,
`users`. `username`,
`chat`.`timestamp`
from `chat` JOIN `users` ON (`chat`.`user_id`=`users`.`user_id`)
ORDER BY `chat`.`timestamp` DESC limit 15";
try{
if($stmt=$mysqli->prepare($sql)){
echo returnJson($stmt);
$stmt->close();
$mysqli->close();
}else{
throw new Exception("An error occured while fetching record data");
}
}catch(Exception $e){
log_error($e, $sql, null);
echo 'fail';
}
}
here is the callback function:
function getChatCallback(data){
var h='';
for(var i=0, l=data.length;i<l;i++){
h+=data[i].username+' says: '+data[i].message+'<span style="color:gray"> at the time '+data[i].timestamp+'</span><br/>';
}
$('.messages').html(h);
setTimeout(getChat,1500);
}
this is the function that will insert the new messages to the database which is called once the user hits enter and it is called also by ajax call:
function putChatData($message,$room,$user_id){
global $mysqli;
$sql = "INSERT INTO `chat`
(`message`,`timestamp`,`room`,`user_id`)
VALUES (?,?,?,?)";
$timestamp = gmdate("Y-m-d H:i:s");
try{
if($stmt=$mysqli->prepare($sql)){
$stmt->bind_param('ssii',$message,$timestamp,$room,$user_id);
$stmt->execute();
$stmt->close();
$mysqli->close();
}else{
throw new Exception("An error occured while fetching record data");
}
}catch(Exception $e){
log_error($e, $sql, null);
echo 'fail';
}
}
the problem now is that I could insert the new messages to the db, but the first function the gets the chat, brings all that chat including the old ones before the user logs in. I tried to use timestamp or the last insert id but didn't work with me.

Answer 1

For this usecase you need to know the reg time and add a WHERE condition to your query
$sql = "SELECT `chat`.`message`,
`users`. `username`,
`chat`.`timestamp`
from `chat` JOIN `users` ON (`chat`.`user_id`=`users`.`user_id`)
WHERE `chat`.`timestamp` > $userRegDate
ORDER BY `chat`.`timestamp` DESC limit 15";