If condition inside the longpolling?

Question

I would like you to only perform longpolling if the if($row["usuario"]=="123"), but I don’t know how to do this because it’s for a chat and any request that someone send to the comic all the chats are affected is affecting other chats.

If I put one if($row["usuario"]=="123"){ executar o longpolling somente nesta linha do bd } already solve, follow my code:

<?php
 $timestart = time();
 $pdo = new PDO('mysql:host=localhost;dbname=carrinho', 'root', 'root');
 
 if(isset($_POST['timestamp'])){
    $timestamp = $_POST['timestamp'];
 }else{
    $pega_time = $pdo->prepare("SELECT NOW() as now");
    $pega_time->execute();
    $row = $pega_time->fetchObject();
    
    $timestamp = $row->now;
 }
 $sql = $pdo->prepare("SELECT * FROM chat WHERE timestamp > '$timestamp'");
 $sql->execute();
 
 $newData = false;
 $notificacoes = array();
 
 while(!$newData && (time()-$timestart)<20){
     $sql->execute();
    
    while($row = $sql->fetchAll(PDO::FETCH_ASSOC)){
        
        $notificacoes = $row;
        $newData = true;

    }
    
    usleep(500000);
 }
 
    $pega_time = $pdo->prepare("SELECT NOW() as now");
    $pega_time->execute();
    $row = $pega_time->fetchObject();
    
    $timestamp = $row->now;
    $data = array('notificacoes' => $notificacoes, 'timestamp' => $timestamp);
    echo json_encode($data);
    exit;
?>