auth.php

<?
class AuthExceptionLogin extends Exception{
    public function __construct($message = "Логина нет в системе", $code = 0x100001, Throwable $previous = null)
    {
        parent::__construct($message, $code, $previous);
    }

}
class AuthExceptionPassword extends Exception{
    public function __construct($message = "Неверный пароль", $code = 0x100002, Throwable $previous = null)
    {
        parent::__construct($message, $code, $previous);
    }

}
class AuthClass{
    public $session_name = false;
    private $authData = array(
        'chexov' => '123456',
        'test' => '909090'
    );

    public function sessionStart($name = false)
    {
        if($name){
            $this->session_name = $name;
            session_name($name);
        }
        if(!session_start()){
            echo 'Не удалось стартовать сессию';
        }
    }

    public function auth($login, $password)
    {
        $login = htmlspecialchars($login);
        $password = htmlspecialchars($password);
        if(isset($this->authData[$login])){
            $passwordBase = $this->authData[$login];
            if($passwordBase === $password){
                $this->setAuth($login, $password);
            } else{
                throw new AuthExceptionPassword();
            }
        } else{
            throw new AuthExceptionLogin();
        }
    }

    public function setAuth($login, $password)
    {
        if($login && $password){
            $_SESSION['is_auth'] = true;
            $_SESSION['password'] = md5($password);
            $_SESSION['login'] = $login;
        }
        session_write_close();
    }

    public function logout()
    {
        $_SESSION = array();
        session_destroy();
        header("Location: ?is_exit=0"); //Редирект после выхода
    }

    public function isAuth()
    {
        return isset($_SESSION['is_auth']) && $_SESSION['is_auth'] == true;
    }

    public function getLogin()
    {
        if($this->isAuth() && isset($_SESSION['login'])){
            return $_SESSION['login'];
        }
        return false;
    }

    public function main()
    {
        $this->sessionStart('FEELK_SESSION');
        if($_POST){
            if(isset($_POST['enter'])){ // хотят войти
                try{
                    $this->auth($_POST['login'], $_POST['password']);
                }catch (AuthExceptionLogin $e){
                    echo $e->getMessage();
                }
                catch (AuthExceptionPassword $e){
                    echo $e->getMessage();
                }
            } elseif (isset($_POST['logout'])){
                $this->logout();
            }
        }
    }

}
?>
<?
    $s = new AuthClass();
    $s->main();
?>
<form action="index.php" method="post">
    <?if(!$s->isAuth()):?>
        <p>Логин: <input type="text" name="login" placeholder="Введите логин" value=""></p>
        <p>Пароль: <input type="password" name="password" placeholder="Введите пароль" value=""></p>
    <?else:?>
        <?echo 'Здравствуйте: '.$s->getLogin();?>
    <?endif;?>
    <div class="btns">
        <?if(!$s->isAuth()):?>
            <input type="submit" value="Войти" name="enter">
        <?else:?>
            <input type="submit" value="Выйти" name="logout">
        <?endif;?>
    </div>
</form>