ho risolto installando il file per le sessioni del php da yast, ora però ottengo 2 warnings:

Warning: session_start(): Cannot send session cookie - headers already sent by (output started at /srv/www/htdocs/prova.php:1) in /srv/www/htdocs/prova.php on line 3

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /srv/www/htdocs/prova.php:1) in /srv/www/htdocs/prova.php on line 3
anche se poi funziona tutto

ecco il codice:
LOGIN.PHP
<?php
session_start();
?>
<HTML>
<HEAD>
<TITLE> Prova sessione </TITLE>
</HEAD>
<BODY>
<?php
if (isset($_SESSION['count'])) {
echo "Hai visitato questa pagina $_SESSION[count] volte";
$_SESSION['count']++;
} else {
echo "Sei un nuovo utente";
$_SESSION['count']=1;
}
?>
</BODY>
</HTML>

LOGOUT.PHP
<?php
session_start();
session_destroy();
?>
<?php
if (isset($_SESSION['count'])) {
echo "$_SESSION[count]";
} else {
echo "sessione distrutta";
}
?>