antispam con somma help me!

[bstefano79]

<?php
$a = 2;
$b = 3;
?>

...

<label><?php echo $a." + ".$b ?> =</label>
<input id="anti" name="anti" type="text" />

comments.php

$risposta=5;
if ($POST['anti'] != $risposta) {
$error_message.=$_REQUEST['r_disc_body'] . "
";
}