If you have an item where you allow users to add comments, how can you pass which item the user is replying too?

I've though of using a hidden field in a form, however this can be easily changed using plugins such as firebug:

<form method="post" action="blah">
<input type="hidden" name="item_id" value="<?php echo $item_id; ?>">
<!-- other form data here -->
<input type="submit" name="submit">
</form>

Or just simply using a session:

$_SESSION['item_id'] = $item_id

Is there a safe way to send the item data in a form?