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

Question

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";
}
}

Answer 1

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.

Answer 2

what is 5 + 7 is your question..
what you can do is:
<?php
$int = 12;
if(!filter_var($12, FILTER_VALIDATE_INT))
{
//your code
}
else
{
echo("Please enter a correct value");
}
?>
I don't know if it's the best solution but this might work..