iFrame Tracking with Adwords & CPA

I have an iFrame setup on my domain which displays a CPA offer.

I'm driving traffic to it via Adwords (despite the T&C).

I'd like to track the sub IDs and keywords all the way from Google to the CPA network.

So far, I have this 3 step code but I'm experiencing problems with it:

<?php
$tracking=$_GET[keyword];
?>

and then the iframe would contain my affiliate link to which I have appended some more code like this:

<iframe src="http://aff.copeac.com/t.asp?id=7793&e=35994&subid=<?php echo $tracking; ?>" >
</iframe>

Then on Google Destination URL, I have been asked to put a link like this:

http://www.mydomain.com/landing-page.html?keyword={keyword}


In theory, this is meant to work.

However, as soon as I place the first bit of code and then visit my landing page, I get these 2 errors:

Notice: Use of undefined constant keyword - assumed 'keyword' in /home/cea/public_html/offer/index.php(353) : eval()'d code on line 2

Notice: Undefined index: keyword in /home/cea/public_html/offer/index.php(353) : eval()'d code on line 2

Any ideas what could be wrong and how it can be fixed?

Appreciate the help,

JH