PHP 5.1 Release Candidate 1 is now available!
If all goes well, this RC will be followed by a release within a couple of weeks.

Some of the key improvements of PHP 5.1 include:

  • PDO (PHP Data Objects) – A new native database abstraction layer
    providing performance, ease-of-use, and flexibility.
  • Significantly improved language performance mainly due to the
    new Zend Engine II execution architecture.
  • The PCRE extension has been updated to PCRE 6.2.
  • Many more improvements including lots of new functionality
    & many bug fixes, especially in regards to SOAP, streams
    and SPL.
  • See the bundled NEWS file for a more complete list of changes.

Everyone is encouraged to download and test this release candidate,
although it is not yet recommended for mission-critical
production use.

Source PHP.NET