if(isset($_POST['login'])) {
    $username = mysql_real_escape_string($_POST['username']);
    $password = $_POST['password'];

        $salt = substr($password, rand(0,30), 10);
    $enc_password = hash('sha512', $salt, $password);
    $q = "SELECT * FROM user WHERE username='$username' AND password='$enc_password'";
        $res = mysql_query($q) or die(mysql_error());

        if(mysql_num_rows($res) < 1 && !empty($username) && !empty($enc_password)) {
            header('Location: user.php?loginerror=');
            exit;
        }

        while($row = mysql_fetch_array($res)) {

        $_SESSION['id']     = mysql_real_escape_string($row['id']);
        $_SESSION['user']   = mysql_real_escape_string($row['username']);
        $_SESSION['flag']   = mysql_real_escape_string($row['flag']);

            header('Location: user.php');
            exit;
        }
    }