if function works, but appears in plaintext on the site too

Hi, I can’t figure out how to get round this, I only tinker with PHP through using WordPress, so I’m sure it’s a pretty simple fix.

I’m trying to display a brief message to adblock users, in place of the ad.

So here’s the portion of code from sidebar.php:

<script>
//Google AdSense stuff..
</script>

if (typeof(window.google_render_ad) == "undefined") {
<div id="adblock">
//My message
</div>
}

But on the page, I see the message, but also the if code before and after.

I tried putting the whole if(){} statement in <?php ?> tags, but that screwed with the whole formatting of the page, and the message didn’t display at all.

Thanks in advance for any tips.