The second alpha (not for use in production) of the PHP 5.6.x series has been officially announced on the main PHP.net site today.

The PHP development team announces the immediate availability of PHP 5.6.0alpha2. This release adds new features and fixes bugs. All users of PHP are encouraged to test this version carefully, and report any bugs in the bug tracking system. [...] PHP 5.6.0alpha2 comes with a number of new features, including: peer certificates are now verified by default when connecting to SSL/TLS servers, an exponentiation operator has been added: **, and output encoding handling [that] has been simplified by using default_charset as the default character encoding.

If you're looking for more information about what's involved in migrating up to this version, check out the upgrade guide or for just the full list of change take a look at the NEWS file. You can download this alpha release from the PHP QA site (Windows binaries here).