On his blog, Ivo Jansch asks the question "What php search engine would you use?" in this blog entry. It looks at some of the search engine systems offered out there and gives some of his opinions on each.

I'm facing a challenge with a customer running a site we built on a hosting environment that is quite restrictive. We used to use htdig to add search functionality, but the hoster doesn't allow execution of binaries.

I've taken a look at several search engine implementations, but so far, none match all requirements.

Among those on the list he's looked into, there's included Zend Framework's lucene search, google, and phpdig. He lists out some of the other requirements that he's looking for and asks for any suggestions that the community might have. Be sure to check out the comments for some great answers already.