If you don't want to use a text file, and you don't want to use mySQL, and if you are using PHP 4:
srand ((float) microtime() * 10000000);
$banner_urls = array (
"banner1.jpg",
"banner2.jpg",
"/advertisements/bork.gif",
"you_get_the_idea.png");
$index = array_rand ($banner_urls, 1);
echo "<img src=/"" . $banner_urls[$index] . "/">";
HTH