After a scratching and searching for a solution with no luck I finally resolved this way:
<form name="form" id="form" method='post' action='nextpage.php'>
<input type="hidden" name="firstname" value="<?php echo $_SESSION['firstname'];?>" />
<input type="hidden" name="lastname" value="<?php echo $_SESSION['lastname'];?>" />
<input type="hidden" name="email" value="<?php echo $_SESSION['email'];?>" />
on my next page I assigned the vars to session using post.
$_SESSION['firstname'] = $_POST['firstname'];
$_SESSION['lastname'] = $_POST['lastname'];
$_SESSION['email'] = $_POST['email'];