Variable not showing up in postbit template

[shridevimoogi]

Hi,

I want to add an ad after the first post.
This I am able to do by adding the below code in postbit template

<if condition="!$GLOBALS['FIRSTPOSTID']"> 
<div class="firstpostad">
Ad Code Here
</div>
</if>

Now the problem is I want some part of the ad code change dynamically based on url.
For this I have written a php code which works fine independently and in header template as well. But not in postbit template.
I tried using $global, then writing in plugin and as a separate php file but in vain.
Please help.
My vb version is 3.8.1 patch level1
My code is as below

<?php



function ad_after_first_post()
{
 $hostname=$_SERVER["SERVER_NAME"];
 $urls = array('test.forums.v3.co.uk','test.forums.accountancyage.com','test.forums.businessgreen.com','test.forums.computeractive.co.uk');
 
 $brand_name['test.forums.v3.co.uk']='vn2.uk';
 $brand_name['test.forums.accountancyage.com']='aa2.uk';
 $brand_name['test.forums.businessgreen.com']='bg2.uk';
 $brand_name['test.forums.computeractive.co.uk']='ca2.uk';
 
foreach($urls as $url)
{
  if ($hostname==$url)
  {
    $brand_var_for_ad=$brand_name[$url];
  }
	
  }
  return $brand_var_for_ad;
}

$ad=ad_after_first_post();




?>