With the form you've made, include the ID in the URL. So the action tag will be action="<?php echo $_SERVER['PHP_SELF'] . '&id= '. $id; ?>".

But first you want to check the ID is save for HTML input via the GET variable:

if(!is_numeric($_GET['id']) OR empty($_GET['id'])) {
die('no valid id');
}

That is a very simple (but ugly) way to do it :)

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Nunchaku! Who doesn't like martial arts? =)