Q:

How do people post on my guestbook without entering the captcha?

I added a textbox to make sure spammers don't post on my guestbook.  I wrote the question in Chinese so it's harder (I hope), but it doesn't seem to work.  As you can see, the spammers still can post.  How is it done?
http://www.badmintontw.com/guestbook.php
Thank you.
The form code:
<script>
$(document).ready(function(){
    $('#guestbook').submit( function(){
        return valid_form();
    });
});

function valid_form()
{
    if ( $('#content').val() == ''){
        alert('沒有留言');
        return false;
    }
    if ($('#sum').val() != 12){
        alert('請輸入正確的數字');
        return false;
    }
 return true;
}

</script>
<h1>留言板</h1>
<p>對本網站有任何想法、問題，歡迎在此留言!</p>
<form name="guestbook" id="guestbook" action="guestbook_process.php" method="post">
    <!--<input type = "hidden" name = "post_id" value = "<?php echo $id; ?>">-->
    <textarea id="content" name = "content" placeholder="留言"></textarea>
    <br />五加七等於多少? <input type="text" name="sum" id="sum">
    <br /><input type="submit" name="submit" id="submit" value="送出">
</form>

And guestbook_process.php contains:
if(isset($_POST['submit'])){
    if($_POST['content'] != ""){
        $guestbook_insert_sql = "insert into badminton.guestbook(guestbook_ip, content, time) values (:guestbook_ip, :content, current_timestamp)";
        $result = $db->prepare($guestbook_insert_sql);
        $result->execute(array( ':guestbook_ip' => $_SERVER['REMOTE_ADDR'],
                                ':content' => $_POST['content']));
        header("Location: guestbook.php");
    }else{
        echo "沒有內容";
    }
}

A:

You need to check the answer to the captcha on the server side (e.g. in your guestbook_process.php). The spammer just doesn't execute your Javascript. Just try to disable javascript in your browser and see for yourself. 

