Changes in database support
PHP Data Objects (PDO) were introduced as a PECL extension under PHP 5.0, and became part of the core PHP distribution in PHP 5.1.x. The PDO extension provides a consistent interface for database access, and is used alongside database-specific PDO drivers. Each driver may also have database-specific functions of its own, but basic data access functionality such as issuing queries and fetching data is covered by PDO functions, using the driver named in PDO::__construct().
Note that the PDO extension, and its drivers, are intended to be built as shared extensions. This will enable straightforward driver upgrades from PECL, without forcing you to rebuild all of PHP.
At the point of the PHP 5.1.x release, PDO is more than ready for widespread testing and could be adopted in most situations. However, it is important to understand that PDO and its drivers are comparatively young and may be missing certain database-specific features; evaluate PDO carefully before you use it in new projects.
Legacy code will generally rely on the pre-existing database extensions, which are still maintained.
Changes in MySQL support
In PHP 4, MySQL 3 support was built-in. With the release of PHP 5.0 there were two MySQL extensions, named 'mysql' and 'mysqli', which were designed to support MySQL < 4.1 and MySQL 4.1 and up, respectively. With the introduction of PDO, which provides a very fast interface to all the database APIs supported by PHP, the PDO_MYSQL driver can support any of the current versions (MySQL 3, 4 or 5) in PHP code written for PDO, depending on the MySQL library version used during compilation. The older MySQL extensions remain in place for reasons of back compatibility, but are not enabled by default.
Changes in SQLite support
In PHP 5.0.x, SQLite 2 support was provided by the built-in sqlite extension, which was also available as a PECL extension in PHP 4.3 and PHP 4.4. With the introduction of PDO, the sqlite extension doubles up to act as a 'sqlite2' driver for PDO; it is due to this that the sqlite extension in PHP 5.1.x has a dependency upon the PDO extension.
PHP 5.1.x ships with a number of alternative interfaces to sqlite:
The sqlite extension provides the "classic" sqlite procedural/OO API that you may have used in prior versions of PHP. It also provides the PDO 'sqlite2' driver, which allows you to access legacy SQLite 2 databases using the PDO API.
PDO_SQLITE provides the 'sqlite' version 3 driver. SQLite version 3 is vastly superior to SQLite version 2, but the file formats of the two versions are not compatible.
If your SQLite-based project is already written and working against earlier PHP versions, then you can continue to use ext/sqlite without problems, but will need to explicitly enable both PDO and sqlite. New projects should use PDO and the 'sqlite' (version 3) driver, as this is faster than SQLite 2, has improved locking concurrency, and supports both prepared statements and binary columns natively.
You must enable PDO to use the SQLite extension. If you want to build the PDO extension as a shared extension, then the SQLite extension must also be built shared. The same holds true for any extension that provides a PDO driver