PDO is a PHP extension that serves as a data access abstraction layer for accessing databases.
The PHP Data Objects (PDO) extension defines a lightweight, consistent interface for accessing databases in PHP. 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.
Via PHP.net PDO