Codice PHP:
/*index.php*/
<?php session_start(); echo $_SESSION['username']; ?>
<!DOCTYPE html>
<html>
    <head>
        <title>index.php</title>
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <meta charset="utf-8">
         <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
        <script src="https://cdnjs.cloudflare.com/ajax/libs/mathjs/3.14.1/math.min.js"></script>
        <link href="config.css" type="text/css" rel="stylesheet">
    </head>

    <body>

        <div id="form-riconoscimento">
            <h4>modulo di riconoscimento</h4>
            <button onclick="generaUsername()">genera username</button>
                <table>
                    <form method="post" action="server.php">

                        <tr>
                            <td><input type="text" name="username" id="username" required></td>
                        </tr>    
                        <tr>    
                            <td><input type="password" name="password" placeholder="password"></td>
                        </tr>
                        <tr>
                            <td><input type="radio" name="opzioneUtente" value="registrareUtente">registrasi</td></tr>
                        <tr>
                            <td><input type="radio" name="opzioneUtente" value="loggareUtente" checked>accedere</td>
                        </tr>
                        <tr>
                            <td><input type="submit" value="invio"></td></tr>
                    </form>
                </table>
        </div>
  

</body>
Codice PHP:
/*server.php*/

<?php
 error_reporting(E_ALL);
session_start();
$host = "localhost";
$username = "root";
$password = "";
$db = "my_laboraccio";
$conex = mysqli_connect($host, $username, $password, $db);


/*variabili di index.php*/
$uname = $_POST['username'];
$pw =  $_POST['password'];
$opzioneUtente = $_POST['opzioneUtente'];

    function registraUtente(){
        global $uname;
        echo $uname;
        global $pw;
        global $conex;
        $sql = "SELECT * FROM utenti WHERE username='$uname'";
        $resource = mysqli_query($conex, $sql);
        $result = mysqli_num_rows($resource);
        echo $result;
        if($result == 0) {
            $sql = "INSERT INTO utenti (id, username, password) VALUES (NULL, '$uname', '$pw')";
            mysql_query($conex, $sql);
            header("Refresh:10; url=http://www.laboraccio.altervista.org/debug/index.php");
              /*echo "nuovo utente creato";
 
            echo "registrazione completata";
        */}
        elseif($result == 1){
              header("Refresh:10; url=http://www.laboraccio.altervista.org/debug/index.php");
            /*echo "username gia in uso, riprova con un altro";*/

           }
    }
    
        function loggareUtente(){
        global $uname;
        global $pw;
        global $conex;
        $sql = "SELECT * FROM utenti WHERE username='$uname' AND password='$pw'";
        $resource = mysqli_query($conex, $sql);
        $result = mysqli_num_rows($resource);
        if($result == 1) {
            $_SESSION['username'] = $uname;
            $_SESSION['password'] = $pw;
            header("Refresh:10 ;url=http://www.laboraccio.altervista.org/debug/mysqli/profilo.php");


        
        }
        elseif($result == 0) {print "utente non valido";  
        header("Refresh:10; url=http://www.laboraccio.altervista.org/debug/mysqli/index.php");
        }

    
    }


if($opzioneUtente == 'registrareUtente'){
    registraUtente();
}
elseif($opzioneUtente == "loggareUtente") {
    loggareUtente();
}

?>
Codice PHP:
<?php
error_reporting(E_ALL);

session_start();
echo $_SESSION['username'];
/*
if(!isset($_SESSION['username'])){
    header("Refresh:10 ;url=http://laboraccio.altervista.org/debug/mysqli/index.php");

}*/
?>
dopo aver fatto il login, passo per la pagina del server e poi una volta arrivato alla pagina del profilo mi da questo errore
Notice: Undefined index: username in /membri/laboraccio/debug/mysqli/profilo.php on line 5

poi se torno al index, mi fa vedere normalmente il mio nome utente.