Je suis debutante et j'aimerais de l aide svp

<?php
class Database
{
    private static $dbName = 'gestioncarriere' ;
    private static $dbHost = 'localhost' ;
    private static $dbUsername = 'root';
    private static $dbUserPassword = '';
     
    private static $cont  = null;
     
    public function __construct() {
        die('Init function is not allowed');
    }
     
    public static function connect()
    {
     
       if ( null == self::$cont )
       {     
        try
        {
          self::$cont =  new PDO( "mysql:host=".self::$dbHost.";"."dbname=".self::$dbName, self::$dbUsername, self::$dbUserPassword); 
        }
        catch(PDOException $e)
        {
          die($e->getMessage()); 
        }
       }
       return self::$cont;
    }
     
    public static function disconnect()
    {
        self::$cont = null;
    }
}
?>

<?php
 session_start();
 require_once 'dbconfig.php';
 
 if(isset($_POST['btn-login']))
 {
  //$user_name = $_POST['user_name'];
  $user_name = trim($_POST['user_name']);
  $user_pass = trim($_POST['user_pass']);
  
  $user_pass = md5($user_pass);
  
  try
  { 
  
   $stmt = $db_con->prepare("SELECT * FROM users WHERE user_name=:user_name");
   $stmt->execute(array(":user_name"=>$user_name));
   $row = $stmt->fetch(PDO::FETCH_ASSOC);
   $count = $stmt->rowCount();
   
   if($row['user_pass']==$user_pass){
    
    echo "ok"; // login
    $_SESSION['user_session'] = $row['user_id'];
   }
   else{
    
    echo "login ou password n'existe pas !!."; // erreur 
   }
    
  }
  catch(PDOException $e){
   echo $e->getMessage();
  }
 }