I created Fusion as a PHP Back to Back User Agent (B2BUA) to show what was possible with the PHP Threads library. The SIP protocol is a very involved protocol and PHP is a very high level language, so it was developed simply as a challenge to see if it could be done.

The software takes a class-based approach to receiving and dispatched incoming SIP messages on a public port. If anyone would like to use a PHP user agent in their software, please get in touch!