Updated Oct 2 2008: The Mission

There is a decided dearth of good PHP open source search engine solutions, but Sphider seems to be the leading candidate these days - "a lightweight web spider and search engine written in PHP, using MySQL as its back end database".

Updated Sept 16 2008

Semantic web search engines ? Social bookmarking run mad ? We shall see. The new search technology is still early in the S-curve and more useful for pop culture subjects than for rule based systems.

From the WikiaSearch project page:

Wikia's search engine concept is that of trusted user feedback from a community of users acting together in an open, transparent, public way.