MEssage Alerte s'ouvre à l'ouverture de la page

if (!empty($_POST)) {
    $errors = array();

    if (empty($_POST['username']) || !preg_match('/^[a-zA-Z0-9_]+$/', $_POST['username'])) {
        $errors['username']= "Votre pseudo ne correspond pas aux caractères valides";
    } else {
        $query = $pdo->prepare("SELECT id FROM user WHERE username = ?");
        $query->execute([$_POST['username']]);
        $u = $query->fetch();
        if ($u) {
            $errors['username'] = 'Pseudo déjà pris';
        }
    }

    if (empty($_POST['password']) || $_POST['password'] != $_POST['password_confirm']) {
        $errors['password']= "Votre mot de passe ne correspond pas ou n'est pas rempli correctement!";
    }

    if (empty($errors)) {
        $query =$pdo->prepare("INSERT INTO user SET username = ?, password = ?");
        $password = password_hash($_POST['password'], PASSWORD_BCRYPT);
        $query->execute([$_POST['username'], $password]);
        die('Compte enregistré');
        $_SESSION['auth'] = $u;
            header('Location: ' . $router->url('blog'));
            exit();
    }
}

<?php
//---------------------------------------------------------------------------------------//
//démarrage des sessions
//---------------------------------------------------------------------------------------//
session_start();

//---------------------------------------------------------------------------------------//
//affichage des erreurs PHP
//---------------------------------------------------------------------------------------//
error_reporting(E_ALL);
ini_set('display_errors', TRUE);
ini_set('display_startup_errors', TRUE);

//---------------------------------------------------------------------------------------//
//Fichier de connexion à la bdd 
//---------------------------------------------------------------------------------------//
require_once 'connexionBdd.php'; // a remplacer par le chemin vers ton fichier de connexion à la bdd


//---------------------------------------------------------------------------------------//
//----- Découpage du code en FONCTIONS -------------//
// tu pourrais d'ailleurs les déplacer dans un fichier à part  et y ajouter une fonction
// pour récupérer les infos de l'utilisateur à partir de son id,
// une fonction pour faire l'authentification .. bref.. tout ce qui concerne le user...
//---------------------------------------------------------------------------------------//

function getUserIdByUserName($username){
  global $pdo;
  try{
    $query = $pdo->prepare("SELECT id FROM user WHERE username = ?");
    $query->execute([$username]);
    return $query->fetch(PDO::FETCH_COLUMN); // https://phpdelusions.net/pdo/fetch_modes#FETCH_COLUMN

  }catch(Exception $e){
    echo "Erreur " : .$e->getMessage();
    exit;
  }
  
}

function addUser($username,$password){
  global $pdo;
  try{
    $query = $pdo->prepare("INSERT INTO user SET username = ?, password = ?");
    $password = password_hash($password, PASSWORD_BCRYPT);
    $result = $query->execute([$username, $password]);
  }catch(Exception $e){
    echo "Erreur " : .$e->getMessage();
    exit;
  }
  return $result;
}


//---------------------------------------------------------------------------------------//
//récupoération PROPRE des variables AVANT de les utiliser
//---------------------------------------------------------------------------------------//
$username = !empty($_POST['username']) ? trim($_POST['username']) : NULL;
$password = !empty($_POST['password']) ? trim($_POST['password']) : NULL;
$password_confirm = !empty($_POST['password_confirm']) ? trim($_POST['password_confirm']) : NULL;

//---------------------------------------------------------------------------------------//
//on vérifie que le formulaire a été envoyé  ( tu peux remplacer le "username" par le name de ton bouton submit si tu veux
//---------------------------------------------------------------------------------------//
if(isset($_POST['username'])){
  $_SESSION['auth'] = NULL;
  $errors = array();
  if (!preg_match('/^[a-zA-Z0-9_]+$/', $username)) {
    $errors['username']= "Votre pseudo ne correspond pas aux caractères valides";
  }else{
     $u = getUserIdByUserName($username);
    if ($u) {
      $errors['username'] = 'Pseudo déjà pris';
    }else{
      
     if (empty($password) || $password != $password_confirm) {
        $errors['password']= "Votre mot de passe ne correspond pas ou n'est pas rempli correctement!";
      } else {
        $result = addUser($username,$password);
        if($result){
          $_SESSION['auth'] = $u;
          echo " Compte enregistré ";
          // header('Location: ' . $router->url('blog'));
          exit();
        }
      }
    }
  } 
}

final class UserTable extends Table
{
    protected $table = "user";
    protected $class = User::class;

public function findByUsername(string $username)
{
    $query = $this->pdo->prepare('SELECT * FROM ' . $this->table . ' WHERE username = :username');
    $query->execute(['username' => $username]);
    $query->setFetchMode(\PDO::FETCH_CLASS, $this->class);
    $result = $query->fetch();

    if ($result === false) {
    throw new NotFoundException($this->table, $username);
    }
    return $result;

    }
    public function createUser(User $user)
    {
        $id = $this->create([
      'username' => $user->getUsername(),
      'password' => $user->getPassword(),
      'is_admin' => 1,
    ]);
        $user->setId($id);
        return $user;
    }

}

$user = new User();
$table = new UserTable(Connection::getPDO());
$pdo = Connection::getPDO();


if (empty($_POST['password'])) {
        $errors['password']= "Votre mot de passe ne correspond pas ou n'est pas rempli correctement!";
    }
    else {
    try {
        $user = $table->findByUsername($_POST['username']);
        if (password_verify($_POST['password'], $user->getPassword()) === true) {
            session_start();
            $_SESSION['auth'] = $user;
        }
        if ($user->getIsAdmin()) {
            header('Location: ' . $router->url('admin_posts'));
            exit();
        } else {
            header('Location: ' . $router->url('blog'));
            exit();
        }
    } catch (NotFoundException $e) {
    }
}
require_once('../views/auth/login_admin.php');

$user = new User();
$table = new UserTable(Connection::getPDO());
$pdo = Connection::getPDO();
$user ->getUsername('username', 'password');
if (empty($_POST['password'])) {
        $errors['password']= "Votre mot de passe ne correspond pas ou n'est pas rempli correctement!";
    } else {
        if (!empty($_POST) && !empty($_POST['username']) && !empty($_POST['password'])) {
            echo ' Vous allez être conntecté';
    }else {
        echo ' merci de recommencer';
    }
            if (password_verify($_POST['password'], $user->password)) {
                $_SESSION['auth'] = $user;
                header('Location: ' . $router->url('blog'));
                exit();
            }
        }
    }
require_once('../views/auth/login_member.php');

$user = new User;
$pdo = Connection::getPDO();
$user->getUsername($user);

$errors = [];
if (!empty($_POST)) {
    $errors = array();
    if (empty($_POST['username']) || !preg_match('/^[a-zA-Z0-9_]+$/', $_POST['username'])) {
        $errors['username']= "Votre pseudo ne correspond pas aux caractères valides";
    } else {
    if ($u) {
    $errors['username'] = 'Pseudo déjà pris';
    }
}
    if (empty($_POST['password']) || $_POST['password'] != $_POST['password_confirm']) {
    $errors['password']= "Votre mot de passe ne correspond pas ou n'est pas rempli correctement!";
}
    if (empty($errors)) {
        die('Compte enregistré');
        $_SESSION['auth'] = $u;
        header('Location: ' . $router->url('blog'));
        exit();
    }
}
require_once('../views/auth/login_register.php');

final class UserTable extends Table
{
    protected $table = "user";
    protected $class = User::class;

public function findByUsername(string $username)
{
    $query = $this->pdo->prepare("SELECT * FROM ' . $this->table . ' WHERE username = ?"');
    $query->execute([ $_POST['username']]);
    $query->setFetchMode(\PDO::FETCH_CLASS, $this->class);
    $result = $query->fetch();

    if ($result === false) {
    throw new NotFoundException($this->table, $username);
    }
    return $result;
    }

    public function addUser($username,$password)
    {
        $query = $this->pdo->prepare("INSERT INTO user SET  username = ?, password = ?");
        $password = password_hash($password, PASSWORD_BCRYPT);
        $query->execute([$username,$password]);
        $query->setFetchMode(PDO::FETCH_CLASS, $this->class);
        $result = $query->fetch();
         if ($result === true) {
            echo 'l \utilisateur est  enregistré';
        } else {
            echo 'merci de recommencer votre enregistrement ';
        }
        return $result;
    }

    public function createUser(User $user)
    {
        $id = $this->create([
      'username' => $user->getUsername(),
      'password' => $user->getPassword(),
      'is_admin' => 1,
    ]);
        $user->setId($id);
        return $user;
    }

}

<?php

namespace App;

use App\Security\ForbiddenException;

class Auth
{
    public static function check()
    {
        if (session_status() === PHP_SESSION_NONE) {
            session_start();
        }
        if (!isset($_SESSION['auth'])) {
            throw new ForbiddenException();
        }
    }
}