Article Rewriter using synonyms PHP

Need an article rewriter in PHP or in other language but can be called by PHP under Linux environment. The rewriter wroks by replacing certain words with synonyms. Note that simply choosing a random synonym is very simple and even I can write such script in 5 minutes. However it WILL NOT work because words are context dependant and the result will be a total bizzaire. Im looking for a slightly more sophisticated solution that can rewrite the article without altering word's context. Database wise, I'm not interested in using third party API unless it is free and supports unlimited queries. So is best to use a local database. In your bid please let me know which public database you are going to use or how you are going to build the database, also how you are going to find synonyms with same context.