I've found a bug with array_rand in PHP 4.2.2; it works in earlier versions, however when tested in newer PHP versions the output never changes.

eg.
srand ((double)microtime()*1000000);
$backgrounds = range (1,28);
$rand_keys = array_rand ($backgrounds, 3);
print $backgrounds[$rand_keys[0]]."~".$backgrounds[$rand_keys[1]]."~".$backgrounds[$rand_keys[2]];

This would always for me produce the same results for me whatever I try.