Valadation in guestbook...

Hay ho guys.

I'm trying to add a valadation field for my guestbook so it stops getting attacked by bots. On the guestbook form, i've addded an extra text box called 'antispam' where posters type in NOTSPAM to allow the post. How and where would I add some code that checks this text box contains NOTSPAM and allow the entery to be posted? If it does not contain NOTSPAM, an error message is displayed?

Anyhoo, this is my guestbook code:

PHP:

<?php
    import_request_variables('p');
    if (isset($firstname) && isset($surname) && isset($email))
    {
        $dberror="";
        $result=add_to_database($firstname,$surname,$email,$comment,$dberror);
        if (!$result)
        {
            print"Error: $dberror<br />";
        }
        else
        {
            print"<center>Thanks for signing <b>$firstname $surname</b></center><br />";
        }
    }
    else
    {
        write_form();
    }
    function add_to_database($f,$s,$e,$c,&$dbe)
    {
        $user="XXXXXXXX";
        $pass="XXXXXXXX";
        $db="guestbook";
        $link=mysql_connect("sql02",$user,$pass);
        if (!$link)
        {
            $dberror="Could not connect to MySQL server";
            return false;
        }
        If (!mysql_select_db($db,$link))
        {
            $dbe="Could not connect to $db";
            return false;
        }

        $query="INSERT INTO guestbook (firstname,surname,email,comment) VALUES ('$f','$s','$e','$c')";
        {
            if (!mysql_query($query, $link))
            {
                $dbe=mysql_error();
                return false;
            }
            return true;
        }
    }
    function write_form()
    {
        print"<background='images/background.png'>";
        print"<form method='post' action='$PHP_SELF'>";
        print"<strong> Firstname:</strong><br>";
        print"<input type ='text' name='firstname'/><br>";
        print"<strong>Surname:</strong><br>";
        print"<input type ='text' name='surname'/><br>";
        print"<strong> E-mail:</strong><br>";
        print"<input type ='text' name='email'/><br>";
        print"<strong>Comment:</strong><br>";
        print"<textarea name='comment' cols='50' rows='5'></textarea><br>";
        print"<strong>Anti-Spam Field</strong><br>";
        print"<strong>Enter NOTSPAM into the text area:</strong><br>";
        print"<input type = 'text' name='antispam'/><br>";
        print"<input type ='submit' value='Sign!'/><input type=\"reset\" name=\"Reset\" value=\"Reset\"/><br>";
        print"</form>";
    }
?>

Sorted. Once I added your bit Neal, I had to add:

PHP:

$antispam = $_POST['antispam'];

...and change...

PHP:

if ($antispam==NOTSPAM)

...to...

PHP:

if ($antispam=="NOTSPAM")

THanks for the help.