problema registrazione

$username= $_POST['username'];
// Recupero la password criptata dal form di inserimento.
$password = $_POST['p'];
$email = $_POST['email'];

// Inserisci a questo punto il codice SQL per eseguire la INSERT nel tuo database
// Assicurati di usare statement SQL 'prepared'.
if ($insert_stmt = $mysqli->prepare("INSERT INTO members (username, email, password) VALUES (?, ?, ?)")) {
$insert_stmt->bind_param('ssss', $username, $email, $password);
// Esegui la query ottenuta.
$insert_stmt->execute();
}

$mysqli = new mysqli("localhost", "my_user", "my_password", "world");

/* check connection */
if ($mysqli->connect_errno) {
printf("Connect failed: %s ", $mysqli->connect_error);
exit();
}

<?php
include_once 'db_connect.php';
include_once 'functions.php';

$error_msg = "";

if (isset($_POST['username'], $_POST['email'], $_POST['p'])) {
// Sanitize and validate the data passed in
$username = filter_input(INPUT_POST, 'username', FILTER_SANITIZE_STRING);
$email = filter_input(INPUT_POST, 'email', FILTER_SANITIZE_EMAIL);
$email = filter_var($email, FILTER_VALIDATE_EMAIL);
if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
// Not a valid email
$error_msg .= '<p class="error">The email address you entered is not valid</p>';
}

$password = filter_input(INPUT_POST, 'p', FILTER_SANITIZE_STRING);
if (strlen($password) != 128) {
// The hashed pwd should be 128 characters long.
// If it's not, something really odd has happened
$error_msg .= '<p class="error">Invalid password configuration.</p>';
}

// Username validity and password validity have been checked client side.
// This should should be adequate as nobody gains any advantage from
// breaking these rules.
//

$prep_stmt = "SELECT id FROM members WHERE email = ? LIMIT 1";
$stmt = $mysqli->prepare($prep_stmt);

// check existing email
if ($stmt) {
$stmt->bind_param('s', $email);
$stmt->execute();
$stmt->store_result();

if ($stmt->num_rows == 1) {
// A user with this email address already exists
$error_msg .= '<p class="error">A user with this email address already exists.</p>';
$stmt->close();
}
$stmt->close();
} else {
$error_msg .= '<p class="error">Database error Line 39</p>';
$stmt->close();
}

// check existing username
$prep_stmt = "SELECT id FROM members WHERE username = ? LIMIT 1";
$stmt = $mysqli->prepare($prep_stmt);

if ($stmt) {
$stmt->bind_param('s', $username);
$stmt->execute();
$stmt->store_result();

if ($stmt->num_rows == 1) {
// A user with this username already exists
$error_msg .= '<p class="error">A user with this username already exists</p>';
$stmt->close();
}
$stmt->close();
} else {
$error_msg .= '<p class="error">Database error line 55</p>';
$stmt->close();
}

// TODO:
// We'll also have to account for the situation where the user doesn't have
// rights to do registration, by checking what type of user is attempting to
// perform the operation.

if (empty($error_msg)) {
// Create a random salt
$random_salt = hash('sha512', uniqid(mt_rand(1, mt_getrandmax()), true));

// Create salted password
$password = hash('sha512', $password . $random_salt);

// Insert the new user into the database
if ($insert_stmt = $mysqli->prepare("INSERT INTO members (username, email, password) VALUES (?, ?, ?)")) {
$insert_stmt->bind_param('sss', $username, $email, $password);
// Execute the prepared query.
if (! $insert_stmt->execute()) {
header('Location: ../error.php?err=Registration failure: INSERT');
}
}
header('Location: ./register_success.php');
}
}