In PHP 5, the SQLite extension and the engine itself are bundled and compiled by default. However, since PHP 5.1.0 you need to manually activate the extension in php.ini (because it is now bundled as shared). Moreover, since PHP 5.1.0 SQLite depends on PDO so it must be enabled too, by adding the following lines to php.ini (in order):
extension=php_pdo.dll
extension=php_sqlite.dll