I'm starting work on a SAPI implementation for the TinyMUX server (http://www.tinymux.org). Basically, it's like a MUD server -- a telnet-based, textual roleplaying game.
What you want is the embed SAPI which can simply be linked against like any other library.
At the moment, the only documentation available (that I know of) are some slides Wez prepared for a conferrence two years ago. Dated perhaps, but extremely relevant and very easy to follow.
http://www.php-kongress.de/2003/slides/internals_track/wez_embedding-php.pdf
-Sara