According to the PHP::Impact blog, the PHP-based replacement for Twitter has finally arrived - identi.ca.

Identi.ca is a microblogging service. Users post short (140 character) notices which are broadcast to their friends and fans using the Web, RSS, or instant messages. It runs on the Free Software Laconica tool.

If you'd like to give it a shot you can register for an account of your own and start posting. It also supports OpenID authentication.