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".

From the site:

FDSE is an easy-to-install search engine for local and remote sites. It returns fast, accurate results from a template-driven architecture.

From the site:

DBpedia is a community effort to extract structured information from Wikipedia and to make this information available on the Web. DBpedia allows you to ask sophisticated queries against Wikipedia and to link other datasets on the Web to Wikipedia data.

Their demo page for the subject Semantic Web, using DBpedia's Linked Data Interface format.