PDO overview

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