Sistema de Login/Registro en POO

<?php
session_start();
include('inc/DbImpl.php');
include('inc/LoginImpl.php');

if(isset($_SESSION['usuario'])){
    header('Location: login.php');
}

if(isset($_POST['submit'])){
    $login = new LoginImpl($_POST['username'],$_POST['password']);
    if($login->verificar()){
	    $_SESSION['usuario'] = $_POST['username'];
	    echo 'Correcto';
	    header('Location: login.php');
    }else{
    	echo 'Incorrecto';
    }
$login->close();
}
?>
<form action="" method="post">
    <input name="username" placeholder="Username"><br>
	<input name="password" placeholder="Password"><br>
    <input name="submit" type="Submit">
</form>
<a href="register.php">Registrate</a>

<?php
session_start();
include('inc/DbImpl.php');
include('inc/RegisterImpl.php');

if(isset($_SESSION['usuario'])){
    header('Location: login.php');
}

if(isset($_POST['submit'])){
    $register = new RegisterImpl($_POST['username'],$_POST['email'],$_POST['password'],$_POST['password2']);
    if($register->addUsername()){
	    header('Location: index.php');
    }
    $register->close();
}
?>
<form action="" method="post">
    <input name="username" placeholder="Username"><br>
	<input name="email" placeholder="Email"><br>
	<input name="password" placeholder="Password"><br>
	<input name="password2" placeholder="Repeat password"><br><br>
    <input name="submit" type="Submit">
</form>

<?php
session_start();
if(isset($_SESSION['usuario'])){
    echo 'Logeado <a href="salir.php">Sal</a>';
}else{
    header('Location: index.php');
}
?>

<?php
session_start();
session_destroy();
header('Location: index.php');
?>

<?php
interface DB {
	public function getDb();
	public function error();
	public function close();
}
class DBImpl {
	
    private $host = '';
    private $tabla = '';
    private $usuario = '';
    private $contrasena = '';
	private $db;

	public function __construct(){
		$this->db = mysqli_connect($this->host,$this->usuario,$this->contrasena,$this->tabla);
	}
	public function getDb(){
		return $this->db;
	}
	public function error(){
		return mysqli_error($this->db);
	}
	public function close(){
		mysqli_close($this->db);
	}

}
?>

<?php
interface Login {
    public function verificar();
}
class LoginImpl extends DBImpl implements Login {

    private $username;
    private $password;
	private $consulta;
    public function __construct($username,$password){
		parent::__construct();
        $this->username = $username;
        $this->password = $password;
    }

    public function verificar(){
        $res = false;
        $this->consulta = mysqli_query(parent::getDb(), "SELECT username,password FROM username WHERE username = '".mysqli_real_escape_string(parent::getDb(), $this->username)."' AND password = '".mysqli_real_escape_string(parent::getDb(), $this->password)."' LIMIT 0,1");
        if($consulta1 = mysqli_fetch_assoc($this->consulta)){
            $res = true;
        }
        return $res;
    }
    
}
?>

<?php
interface Register {
    public function getUsername();
    public function getEmail();
    public function addUsername();
}
class RegisterImpl extends DBImpl implements Register {

    private $username;
    private $email;
    private $password;
    private $password2;
    public function __construct($username,$email,$password,$password2){
		parent::__construct();

        //Validar
        $this->checkUsername($username);
        $this->checkEmail($email);
        $this->checkPassword($password,$password2);

        $this->username = $username;
        $this->email = $email;
        $this->password = $password;
        $this->password2 = $password2;
    }

    private function checkUsername($username){
        $this->consulta = mysqli_query(parent::getDb(), "SELECT username FROM username WHERE username = '".mysqli_real_escape_string(parent::getDb(), $username)."' LIMIT 0,1");
        if($consulta1 = mysqli_fetch_assoc($this->consulta)){
            throw new Exception("Usuario ya existe");
        }
        if(empty($username)){
            throw new Exception("Usuario esta vacio");
        }
    }
    private function checkEmail($email){
        $this->consulta = mysqli_query(parent::getDb(), "SELECT email FROM username WHERE email = '".mysqli_real_escape_string(parent::getDb(), $email)."' LIMIT 0,1");
        if($consulta1 = mysqli_fetch_assoc($this->consulta)){
            throw new Exception("Email ya existe");
        }
        if(empty($email)){
            throw new Exception("Email esta vacio");
        }
    }
    private function checkPassword($password,$password2){
        if($password != $password2){
            throw new Exception("Contraseñas incorrectas, no son iguales");
        }
        if(empty($password)){
            throw new Exception("Password esta vacio");
        }
        if(empty($password2)){
            throw new Exception("Password2 esta vacio");
        }
    }
    
    public function getUsername(){
        return $this->username;
    }
    public function getEmail(){
        return $this->email;
    }

    public function addUsername(){
        $res = false;
        $this->consulta = mysqli_query(parent::getDb(), "INSERT INTO username(username,email,password) VALUES ('".mysqli_real_escape_string(parent::getDb(), $this->username)."','".mysqli_real_escape_string(parent::getDb(), $this->email)."','".mysqli_real_escape_string(parent::getDb(), $this->password)."')");
        if($consulta1 = mysqli_fetch_assoc($this->consulta)){
            $res = true;
        }
        return $res;
    }
}
?>