After registering (in PDO), direct the page to index.html

I'm using HTML and PHP. The connection to the bank works, it registers the entered data and everything. Next, I open index.html, it has the option that takes me to the registration page:

<li><a href="http://localhost/middleearth/form-add.php">REGISTRAR</a></li>

This form-add.php shows me the form and register, and in a file called add.php, I open the connection for insertion in the bank. In this same add.php, I have:

header('Location: register_ok.php');

This register_ok.php informs you that the registration has been successful and gives the link to return to index.html:

<div id="options" class="container text-center">
<h2>CADASTRO</h2>
<p>Cadastro efetuado com sucesso! </p>
<a href="file:///C:/xampp/htdocs/middleearth/personagens.html">Ir para a Tela Inicial</a>
<br>

That's where my problem lives. I know it can be a very stupid thing to solve, but I just can not get it back to index.html so he can go back home and ask for login. I click on the link and it does not open. I have already tried using the header as well and nothing. The issue is, .php files accessed by the localhost / project folder do not seem to support this targeting for .html files. Can someone give me strength?