How to create random, but unique IDs?

[eluser]Steven Ross[/eluser]
Thanks, but those IDs are 13 characters long and in hex, i.e. the contain letters a-f. I'd prefer a purely numeric ID of no more than 8 digits.

Also, from the comments on the manual page:
"this function simply returns the UNIX timestamp with added microsecond counter as a hex number; it's more or less just microtime(), in hexit form"
So, IDs will still be in order small -> large, not random. You can address that with the "added entropy" parameter, but that makes numbers 23 characters - definitely too long for me.