The RFC 4122 describes five versions of UUID, but none of them is optimized to be used as a primary key in a relational database we are so fond of.

Brad Peabody proposed Version 6:

TL;DR: ‘Version 6’ UUIDs have the date/time encoded from high to low bytes (bit-shifting around the version field in order to preserve its location) and thus sort correctly by time when treated as an opaque bunch of bytes; the clock sequence can be used to avoid duplicates generated at the same time; it’s okay to use random data from a good PRNG in place of the MAC address; the rest is the same as RFC 4122.

Pity I didn’t found any PHP implementation, needed to write one myself. Go and grab it from Packagist as mikemix/php-uuid-v6. Hope it helps someone!

Hopefully this proposal will make its way through the IETF process some day! Ben Ramsey already pledged his help.