如何使用AJAX，PHP和MySQL实时发送通知？

I would like to make real-time notification on my website. I have the notification bar:

<div class="alert alert-info alert-with-icon" data-notify="container">
  <button type="button" aria-hidden="true" class="close">
     <i class="nc-icon nc-simple-remove"></i>
  </button>
  <span data-notify="icon" class="nc-icon nc-bell-55"></span>
  <span data-notify="message"><h6>TEXT HERE</h6></span>
</div>

I would like to place my text in the space provided above.

I created a JS function to call a PHP file that will read in the database the last message.

JS code in index.php :

function charger() {

      setTimeout( function(){
        $.ajax({
          url : "charger.php",
          type : GET,
          success : function(html){
            $("h6").prepend(html);
          }
        });

        charger();

      },5000);
    }

    charger();

PHP File in charger.php :

<?php

  $servername = "localhost";
  $username = "scanner";
  $password = "valentin";
  $dbname = "Scanner3D";

  try {
    $bdd = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
    $bdd->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
  }
  catch(PDOException $e)
  {
  }

  // Recuperation des notifications du Scanner
  $requete = $bdd->query('SELECT message FROM notifications ORDER BY id DESC');

  $messages = null;

  while($donnees = $requete->fetch()){
    $messages = $donnees['message'];
  }

  echo $messages;

?>

But I can not display the text I read in my database in my notification location.