random banner code - what am I doing wrong?

[Lpspider]

I found some code on another forum that's supposed to show a random banner within vbulletin. People posting in the thread got it to work, but I'm stuggling. If someone wouldn't mind taking a look and telling me what I'm doing wrong, I'd greatly appreciate it.

I created a new plugin, with the hook location of "global_start" with this code:

$banner[1] = "<center><a href=\"http://advertiser-edit.com/?utm_source=website.org&utm_medium=banner&utm_campaign=websiteleaderboard\"><img src=\"http://www.website.org/leaderboard_books_design.jpg\"></a></center>"; 
$banner[2] = "<center> 
<script type=\"text/javascript\"><!-- 
google_ad_client = \"ca-pub-numbersremoved\"; 
google_ad_slot = \"4591232780\"; 
google_ad_width = 728; 
google_ad_height = 90; 
//--> 
</script> 
<script type=\"text/javascript\" 
src=\"http://pagead2.googlesyndication.com/pagead/show_ads.js\"> 
</script></center>"; 

$rotator = rand(1, 2); 

$bannerrotator = $banner[$rotator];

I've tried placing $bannerrotator in the bottom of the navbar template and also in the ad location in the ads section, but neither work. Instead I get "$bannerrotator" where the ad should appear. What am I doing wrong?

Thanks guys.