<?php
$secret_word = 'salt-baked squid';
$id = 31415926;
$hash = md5($secret_word.$id);
print<<<_HTML_
<form method="POST" action="save.php">
<input type="hidden" name="id" value="$id">
<input type="hidden" name="hash" value="$hash">
... other form elements ...
</form>
_HTML_;
?>