copy and past this code into your php page where
you want the random banner shown.

Title: PHP Random Banner
URL: http://www.mlinks.net/~bas/scripts/index.html
Download URL: http://www.mlinks.net/~bas/scripts/random_banner.zip
Version: 1.0
Platform: Unix, Win 9x/NT/2000
File Size: 1kb
Price: Freeware
Category: PHP/Scripts and Programs 
Snapshot: Display a random banner from a list in with php.
Description: Place the PHP random banner code anywhere on your PHP site to have the banners selected at random from a list that you specify.
Author: ZUg Scripts
Contact Email: septik@onebox.com

<?php   
$bannerlist ="banner.list";    
$readf = fopen($bannerlist, "r")
	while (!feof($readf)) 
	{       
		$line = fgets($readf, "1000");
		$line = trim($readf);
		$rand_banner[] = $line;      
	}     
	srand ((double) microtime() * 18273645);     
	$number = array_rand($rand_banner, 1);     
	print $rand_banner[$number];
?>