Sphinx, Solr, and Lucene are popular technologies for building search indexes and engines. You need to keep in mind though that indexing information is only one component and that you'll probably also want a spidering system. I've not come across any decent options that were open-source and provided both features.
Saw this in marketplace...maybe it would be good for you. https://marketplace.digitalpoint.com/simple-php-search-engine.1660/item