Sphinx is a full-text search engine, distributed under GPL
version 2. a standalone search engine, meant to provide fast,
size-efficient and relevant fulltext search functions to other
applications. Sphinx was specially designed to integrate well with SQL
databases and scripting languages. Currently built-in data source
drivers support fetching data either via direct connection to MySQL,
PostgreSQL, or from a pipe in a custom XML format.
What is great with Sphinx that it came with a pure-PHP searchd client API, this will make its integration with PHP application much more easier. In addition to many features that make Sphinx a very interesting alternative to an open source SQL full-text search engine.
Sphinx is Open Source under GPL license but Commercial license is also available for embedded use.