Might I suggest using SQLite as the DB for the"file format". It's a single file db that performs well for read heavy workloads, is single file, so easy to distribute, easily usable from PHP and just about any other programming language, and could easily be used to load a regular server based db (obv depending on how the schema is designed). Also multi-platform, so should keep everybody happy irrespective of what OS you use.
I forgot to mention, SQLite also supports full text indices via the ft4 virtual table type