nukeevangelist writes:  

The PHP development team is proud to announce the release of PHP 5.1.0. Some of the key features of PHP 5.1.0 include:

A complete rewrite of date handling code, with improved timezone support.
Significant performance improvements compared to PHP 5.0.X.
PDO extension is now enabled by default.

Over 30 new functions in various extensions and built-in functionality.
Bundled libraries, PCRE and SQLite upgraded to latest versions.
Over 400 various bug fixes.
PEAR upgraded to version 1.4.5
In addition to new features, this release includes a number of important security fixes and we recommend that all users of PHP 5.0 and early adopters of PHP 5.1 betas upgrade to this release as soon as possible. The full list of changes in PHP 5.1.0 is available in the complete PHP V. 5xx Changelog [untill today]

some of the more major changes in PHP V. 5: (vgl. die Changelog)