[From MySQL AB]
PDO is a PHP extension providing a data-access abstraction layer that can be used with a variety of databases. This gives you the flexibility of changing the database backend without having to alter your access methods. Even if you use MySQL exclusively, PDO can provide advantages; you can use the same data-access methods regardless of the MySQL version. This does away with the need for using the standard MySQL extension with older versions of MySQL and using the MySQLi extension for later versions. An additional advantage is the ability to use object-oriented code regardless of the version of MySQL.
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