Ошибки:
[Sat May 13 20:12:49.957128 2017] [:error] [pid 31888] [client 188.163.68.144:25652] PHP Notice:  Undefined index: email in /var/www/ullu/ullu.dev/controllers/Registration.php on line 19
[Sat May 13 20:12:49.957186 2017] [:error] [pid 31888] [client 188.163.68.144:25652] PHP Notice:  Undefined index: username in /var/www/ullu/ullu.dev/controllers/Registration.php on line 20
[Sat May 13 20:12:49.957194 2017] [:error] [pid 31888] [client 188.163.68.144:25652] PHP Notice:  Undefined index: password in /var/www/ullu/ullu.dev/controllers/Registration.php on line 21
[Sat May 13 20:12:49.957251 2017] [:error] [pid 31888] [client 188.163.68.144:25652] PHP Fatal error:  Uncaught Exception: Post are empty in /var/www/ullu/ullu.dev/controllers/Registration.php:35\nStack trace:\n#0 /var/www/ullu/ullu.dev/views/client/registration.php(11): controllers\\Registration::createNewUser()\n#1 /var/www/ullu/ullu.dev/controllers/AController.php(12): include('/var/www/ullu/u...')\n#2 /var/www/ullu/ullu.dev/controllers/Registration.php(43): controllers\\AController->render('client/registra...', Array)\n#3 [internal function]: controllers\\Registration->registration()\n#4 /var/www/ullu/ullu.dev/router/Router.php(88): call_user_func_array(Array, Array)\n#5 /var/www/ullu/ullu.dev/router/Router.php(58): router\\Router::executeAction()\n#6 /var/www/ullu/ullu.dev/index.php(24): router\\Router::dispatch()\n#7 {main}\n  thrown in /var/www/ullu/ullu.dev/controllers/Registration.php on line 35

Обработчик
public static function createNewUser()
    {
        $email    = $_POST['email'];
        $username = $_POST['username'];
        $password = $_POST['password'];

        if(isset($email) && isset($username) && isset($password)) {
            $user = new User();
            $user->setEmail($email)
                ->setPassword($password)
                ->setName($username)
                ->send();
            unset($_POST['email'], $_POST['username'], $_POST['password']);

            Header('Location: /auth');
        }
    }

Форма

<form action="<?php \controllers\Registration::createNewUser() ?>" method="post">
        <label for="username__registration">username</label>
        <input type="text"
               name="username"
               id="username__registration"
               required>


        <label for="email_registration">email</label>
        <input
            type="email"
            name="email"
            id="email_registration"
            required>


        <label for="password_registration">password</label>
        <input
            type="password"
            name="password"
            id="password_registration"
            required>

        <input
            type="submit"
            name="submit"
            id="submit_registration">
    </form>