SQLite is a fully functional relational database system that does not use the traditional client/server database architecture. For example, MySQL has a server running on a machine somewhere, and a client (in the form of PHP, in our examples) connects to that server to perform queries. SQLite, on the other hand, works on local files, with no database server required—when you run queries using SQLite, they are translated into operations on the local files.
From PHP 5 onward, SQLite is bundled and enabled by default, which means that everyone, everywhere, will have it by default. If you are writing an application that needs a data store, you no longer need to worry whether they have Oracle or Microsoft SQL Server installed or, indeed, whether they have any database server installed at all.