The SQLite3 extension is enabled by default as of PHP 5.3.0. It's possible to disable it by using --without-sqlite3 at compile time.
Windows users must enable php_sqlite3.dll in order to use this extension. This DLL is included with Windows distributions of PHP as of PHP 5.3.0.
Note:
This extension was briefly a PECL extension but that version is only recommended for experimental use.