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
» http://pecl.php.net/. Windows binaries are
built regularly, and you can obtain the binaries for PECL extensions built
against PHP 5.1.x from
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 CVS at » http://cvs.php.net/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,
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.