Can’t create an user in utenti using PHP registration form [duplicate]

I’ve got an HTTP ERROR 500 after the registration in index.php using PHP 8.3.12 and MySQL 9.0.1, i’ve made 1 file and folder with 2 files:

index.php

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title></title>
</head>

<body>
    <form action="./php/register.php" method="POST">
        <h2>Registrati</h2>

        <div>
            <label for="email">Email</label>
            <input type="email" name="email" id="email">
        </div>

        <div>
            <label for="username">Username</label>
            <input type="text" name="username" id="username">
        </div>

        <div>
            <label for="password">Password</label>
            <input type="password" name="password" id="password">
        </div>

        <input type="submit" value="invia">

    </form>
</body>

</html>

Folder php:

register.php

<?php

require_once('./config.php');

$email = $connessione->real_escape_string($_POST['email']);
$username = $connessione->real_escape_string($_POST['username']);
$password = $connessione->real_escape_string($_POST['password']);

$sql = "INSERT INTO utenti (email, username, password) VALUES ('$email', '$username', '$password')";
if ($connessione->query($sql) === true) {
    echo "registrazione avvenuta";
} else {
    echo "errore registrazione $sql" . $connessione->error;
}

?>

config.php

<?php
$host = "127.0.0.1";
$user = "root";
$password = "rootroot";
$db = "testMySQL";

$connessione = new mysqli($host, $user, $password, $db);

if ($connessione === false) {
    die("errore" . $connessione->connect_error);
}

?>

the connection in config.php with the database went smoothly

Ive tried to:

Change in register.php the

require_once('./config.php');

into

require_once('./php/config.php');

but still not working