get-chat.php

<?php 
    session_start();
    if(isset($_SESSION['unique_id'])){
        include_once "config.php";
        $outgoing_id = mysqli_real_escape_string($conn, $_POST['outgoing_id']);
        $incoming_id = mysqli_real_escape_string($conn, $_POST['incoming_id']);
        $output = "";

        $sql = "SELECT * FROM messages
        LEFT JOIN users ON users.unique_id = messages.outgoing_msg_id 
        WHERE (outgoing_msg_id = {$outgoing_id} AND incoming_msg_id = {$incoming_id})
        OR (outgoing_msg_id = {$incoming_id} AND incoming_msg_id = {$outgoing_id}) ORDER BY msg_id ";//ASC";

        $query = mysqli_query($conn, $sql);
        if(mysqli_num_rows($query) > 0){
            while($row = mysqli_fetch_assoc($query)){
                if($row['outgoing_msg_id'] === $outgoing_id){
                    $output .='<div class="chat-outgoing">
                               <div class="details">
                                   <p>'. $row['msg'] .'</p>
                               </div>
                               </div>';
                }else{//he is a msg receiver
                    $output .='<div class="chat-incoming">
                               <img src="php/images/'. $row['img'] .'" alt="">
                               <div class="details">
                                   <p>'. $row['msg'] .'</p>
                               </div>
                               </div>';

                }
            }
            echo $output;
        }
    }else{
        header("../login.php");
    }
?>