Among the changes found right now for PHP 5.6 include support for new MIME types in the built-in command-line web-server, internal operator overloading support, support for uploads greater than 2GB, reduced _POST data memory usage by 200~300%, dedicated syntax for variadic functions, improved multi-threaded scalability, and constant scalar expressions.
Details on the PHP 5.6 variadic functions can be found via this Wiki page and the constant scalar expressions can be found via this web-page. The improved multi-threaded scalability come via using emalloc, efree, and estrdup.
Some of the non-core PHP 5.6 changes worth noting include support for the CryptoPro S-box GOST hash algorithm within the PHP hashing component, new optimizations inside OPcache, certificate fingerprint support inside the OpenSSL component, session improvements, and many other changes.
More details on the committed changes for PHP 5.6 can be found via the NEWS file in Git. Expect the official PHP 5.6 release within the next few months (at the time of posting I have yet to see a firm release plan).