Erik is close - this module actually provides a PHP interface to the proprietary Muscat 3.6 system which OpenMuscat was originally being written to replace.
OpenMuscat had a rather different API to Muscat 3.6.
BrightStation (the company which owned Muscat) closed down the OpenMuscat project and later ceased trading, but OpenMuscat lived on, renamed Xapian due to complaints about trademark infringement from BrightStation. Xapian has its
own PHP bindings.
I'd be suprised if this module is used by anyone now. I don't think Muscat 3.6 has been available to buy for over 5 years, and even back then most external developers used a higher level API than the one which this module wraps.
Olly Betts (former BrightStation employee and current Xapian developer)