ITema, Inc. today released Blackbird, its PHP enterprise service bus (ESB), to the Open Source community under the GNU General Public License (GPL). Blackbird allows PHP developers to rapidly develop loosely-coupled software applications, allowing them to leverage PHP's development speed and ease of use for application integration tasks.

It works well with Apache ServiceMix (via a common message queue) and has an "extensive collection of APIs, protocol handlers and XML/string parsing routines". The software is available for download and is released under the GNU General Public License as free for both personal and business use. There is also commercial support available as well as a forum to help you get the answers you need.

See also this post from PHPGeek.com