En inglés, copiado de PHP 5.3.0 Release Announcement y visto en el blog de Lester Chan.

The PHP development team is proud to announce the immediate release of PHP 5.3.0. This release is a major improvement in the 5.X series, which includes a large number of new features and bug fixes.

The key features of PHP 5.3.0 include:

• Support for namespaces
• Late static binding
• Lambda Functions and Closures
• Syntax additions: NOWDOC, ternary short cut “?:” and jump label (limited goto), __callStatic()
• Under the hood performance improvements
• Optional garbage collection for cyclic references
• Optional mysqlnd PHP native replacement for libmysql
• Improved Windows support including VC9 and experimental X64 binaries as well as portability to other supported platforms
• More consistent float rounding
• Deprecation notices are now handled via E_DEPRECATED (part of E_ALL) instead of the E_STRICT error level
• Several enhancements to enable more flexiblity in php.ini (and ini parsing in general)
• New bundled extensions: ext/phar, ext/intl, ext/fileinfo, ext/sqlite3, ext/enchant
• Over 140 bug fixes and improvements to PHP, in particular to: ext/openssl, ext/spl and ext/date

This release also drops several extensions and unifies the usage of internal APIs. Users should be aware of the following known backwards compatibility breaks:

• Parameter parsing API unification will cause some functions to behave more
  or less strict when it comes to type juggling
• Removed the following extensions: ext/mhash (see ext/hash), ext/msql, ext/pspell (see ext/enchant), ext/sybase (see ext/sybase_ct)
• Moved the following extensions to PECL: ext/ming, ext/fbsql, ext/ncurses, ext/fdf
• Removed zend.ze1_compatibility_mode

For users upgrading from PHP 5.2 there is a migration guide available here, detailing the changes between those releases and PHP 5.3.0.

For a full list of changes in PHP 5.3.0, see the ChangeLog.