prova a cambiare la prova.php con
codice:
#pagina prova.php
<?php
session_start();
?>
<html>
<head>
<title></title>
</head>
<body>
<?php
$_SESSION['param'] = "ciao mondo";
?>
<form action="prova2.php" method="post">
<input type="submit" value="Send"></form>
</body>
</html>