Extensions that are gone from the PHP core

One of the first things you’re likely to notice
when you download PHP 5.1.x is that several of the older extensions
have disappeared. Those extensions that are still actively
maintained are available in the PHP Extension Community Library
(PECL), at » https://pecl.php.net/.

Modules in PECL that are not actively maintained
(i.e. have not been supported for some time, have no active
maintainer working on them currently, and do not have any PECL
package releases), are still available in SVN at » https://svn.php.net/viewvc/pecl.
However, unreleased PHP modules are by their nature unsupported,
and your mileage may vary when attempting to install or use
them.

Class constants in new PHP 5.1.x extensions

The Zend Engine 2.1 API allows extension developers
to declare class constants in object oriented extensions. New
extensions written for PHP 5.1.x, including SPL, PDO, XMLReader and date, have their constants in the format
PDO::CLASS_CONSTANT rather than in
the C format PDO_CLASS_CONSTANT in
order to minimise pollution of the global namespace in PHP.