How to to force the templates to parse a php script and then call it like '$script'

[Coldhands]

This is what I have in my phpinclude_start file in the template section of the admin cp:

// Example of how to include a seperate file:
// ob_start();
// require("yourheader.html");
// $header = ob_get_contents();
// ob_end_clean();
$numrows = 19;
if ($numrows > 1)
{$numrows = $numrows-1;
mt_srand((double)microtime()*1000000);
$bannum = mt_rand(0, $numrows);
}
else $bannum = 0;
$banneraddress="/home/actuary/public_html/banners/banner".$bannum.".html";
$bannerurl = file_get_contents($banneraddress);

and when I want one of my 19 banners to show up I used to type:

$bannerurl

and one of my banners would show up, but now I get nothing. Is there a change in vB 3.5 with the syntax that I'm missing or is there a better way to server up random html code in the header that I don't know about?