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/.
|ext/fam||Not actively maintained|
|ext/ircg||Not actively maintained|
|ext/mnogosearch||Not actively maintained|
|ext/oracle||ext/oci8 or ext/pdo_oci|
|ext/ovrimos||Not actively maintained|
|ext/pfpro||Not actively maintained|
|ext/yp||Not actively maintained|
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 » http://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.
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.