it looks to me like
$bannerAdTotals = $bannerCounter - 1;
makes $bannerAdTotals == 1, so you're then trying to pull a random number from 1 to 1:
$bannerPicked = mt_rand(1, $bannerAdTotals);
I might do this as follows:
$bannerCode = array();
$bannerCode[] = "Some URL";
$bannerCode[] = "Another URL";
$bannerPicked = mt_rand(0, count($bannerCode) - 1);
$bannerAd = $bannerCode[$bannerPicked];
Note: As of PHP 4.2.0, there is no need to seed the random number generator with srand() or mt_srand() as this is now done automatically.