The PHP Data Objects (PDO) extension defines a lightweight, consistent interface
for accessing databases in PHP. Each database driver that
implements the PDO interface can expose database-specific
features as regular extension functions. Note that you cannot
perform any database functions using the PDO extension by
itself; you must use a database-specific
PDO driver to access a database server.
PDO provides a data-access abstraction layer, which means that, regardless of which database you're using, you use the same functions to issue queries and fetch data. PDO does not provide a database abstraction; it doesn't rewrite SQL or emulate missing features. You should use a full-blown abstraction layer if you need that facility.
PDO ships with PHP.