There were some changes in PHP 5 regarding databases (MySQL and SQLite).
In PHP 5 the MySQL client libraries are not bundled, because of licence problems and some others. For more information, read the FAQ entry.
There is also a new extension, MySQLi (Improved MySQL), which is designed to work with MySQL 4.1 and above.
Since PHP 5, the SQLite extension is built-in PHP. SQLite is embeddable SQL database engine and is not a client library used to connect to a big database server (like MySQL or PostgreSQL). The SQLite library reads and writes directly to and from the database files on disk.