<form action="' . $_SERVER['REQUEST_URI'] . ' " method="post" >
    <div>
    <label for="username">Логин<strong>*</strong></label>
    <input type="text" name="username" value="' . (isset($_POST['username']) ? $username : null) . '">
    </div>

    <div>
    <label for="password">Пароль <strong>*</strong></label>
    <input type="password" name="password" value="' . (isset($_POST['password']) ? $password : null) . '">
    </div>

    <div>
    <label for="email">E-mail <strong>*</strong></label>
    <input type="text" name="email" value="' . (isset($_POST['email']) ? $email : null) . '">
    </div>

    //остальные поля
    <input type="submit" name="submit" value="Зарегистрироваться">