PHP 5.6 Alpha 2 brings an expoential right associative operator (**), a new mysqli_get_links_stats() function, and upgrade to PCRE. These changes build upon the PHP 5.6 Alpha 1 changes from late January that have more MIME types being supported by the CLI web-server, improvements to empty string handling, support for internal operator overloading, reduced _POST data memory usage by 200~300%, a dedicated syntax for variadic functions, implemented constant scalar expressions, OPcache optimizations, OpenSSL improvements, new session functions, and many bug-fixes. Those interested in learning more about all of the changes for the upcoming PHP 5.6 release can read this NEWS blob in Git. Other PHP 5.6 features have also been already talked about on Phoronix.
Details as it affects programmers in moving to PHP 5.6 is covered via the PHP migration manual.
PHP 5.6 Alpha 2 was released last week and more details on its release can be found via php.net. It will be interesting to see how PHP 5.6 compares to Facebook's greatly-advancing HipHop Virtual Machine with HHVM being an alternative PHP implementation.