PHP Random Image Script

[Joe 90]

<?php
$fileList = array();
$folder = ".";
$handle = opendir($folder);
while (
false !== ( $file = readdir($handle) )
) {
if (
substr($file, -4) == ".gif" ||
substr($file, -4) == ".jpg"
) {
$fileList[count($fileList)] = $file;
}
}
closedir($handle);
$randNum = rand( 0, (sizeOf($fileList) -1) );
if (
substr($fileList[$randNum], -4) == ".gif"
) {
header ("Content-type: image/gif");
} elseif (
substr($fileList[$randNum], -4) == ".jpg"
) {
header ("Content-type: image/jpeg");
}
readfile($fileList[$randNum]);
?>

So, I've got this script running and it displays a random image okay, but it appears that cache can cause the same image to be displayed once, and then again and again for the remainder of the instances where the script is called.
http://www.shuwarriors.net

I'm no programmer so don't know enough about php - is there a way to limit the amount of times 1 image can be posted - or some other kind of fix?
thanks.