php how to know that a click came from google
<p>My adsense ad have a dedicated land page.</p>

<p>I want to show the content <strong>only to those who came through that ad</strong>.</p>

<p>The page is coded with PHP so I'm using <code>$_SERVER['HTTP_REFERER']</code>.</p>

<p>Two questions here:</p>

<ol>
<li><p>Is there a better alternative to <code>$_SERVER['HTTP_REFERER']</code> ?</p></li>
<li><p>To what strings/domains should I compare the referrer's domain (I'll handle extracting it)? I mean, I'm guessing that google has more than one domain they're using for the ads, or not? There's doubleclick.com.... any other domain? How can I check it, besides try/fail?</p></li>
</ol>
