<!--  short reproducible error script
srand ((double) microtime() * 10000000);
$count = sizeof($files);
$rand_keys = array_rand ($files, $count);
reset ($files);
while (list ($key, $val) = each ($rand_keys)) {
  echo "$key -> $val<br>\n";
}
//-->

The nature of the bug is in tryign to create a random distribution of the entire array.  The function works perfectly provided the second parameter to the array_rand function is less than the total number of elements.  If the second parameter is equal to the total number of elements i nthe array, then the result of array_rand is identical to the initial array.