ciao ora funziona ma non so perchè perde la variabile che mi serve ($username) passando da una pagina all'altra.
Ho aggiornato l'esempio che potete verere qui :

myscript72.altervista.org/test/index.php

mentre questo è il codice aggiornato.

header.php
Codice PHP:
<table border="1" height="90">
<tr>
    <td width="200" valign="top">[url="index.php"]index.php[/url]
    

    

    </td>
    <td width="400">[url="logout.php"]logout.php[/url]
    
    </td>
    <td width="200" valign="top">
    
    <?
    //Start session
    session_start();

    print_r($_SESSION);
    
    if(empty($_SESSION['SESS_MEMBER_ID']) ) {
     include ('login.php');
    } else { 

        echo "Ciao $username";
      } 
    ?>
    </td>
</tr>
</table>
login.php

Codice PHP:
<?php
    
    session_start();

    $username = $_POST['username'];
    $password = $_POST['password'];

    include("db_vars.inc");

    //Sanitize the value received from login field
    //to prevent SQL Injection
    if(!get_magic_quotes_gpc()) {
        $username=mysql_real_escape_string($_POST['username']);
    }else {
        $username=$_POST['username'];
    }


    //Create query
    $qry="SELECT iduser FROM users WHERE username='$username' AND password='".md5($_POST['password'])."'";
    $result=mysql_query($qry);
    //Check whether the query was successful or not
    if($result) {
        if(mysql_num_rows($result)>0) {
            //Login Successful
            $member=mysql_fetch_assoc($result);
            $_SESSION['SESS_MEMBER_ID']= $member['iduser'];

            session_write_close();
            echo "ciao $username
";

        }else {

?>

<form method="post" action="<?php echo $_SERVER['PHP_SELF'] ?>">
[b]Login utente[/b]


Username:<input type="text" name="username" />

Password:<input type="text" name="password" />


<input type="submit" />
</form>

<?
        }
     }                                      
?>