See: http://php.net/index.php#id2014-08-28-1

The PHP Development Team announces the immediate availability of PHP 5.6.0. This new version release comes with new features, some backward incompatible changes and many improvements.

The main features of PHP 5.6.0 include:

Constant scalar expressionss.
Variadic functions, and argument unpacking syntax
Exponentiation operator.
Function and constants imports with the use keyword.
phpdbg as an interactive integrated debugger SAPI.
php://input attribute changes as well as POST data parsing mechanism.
GMP objects now support operator overloading.
Files larger than 2 gigabytes in size are now accepted.
For a full list of new features, you may read our migration guide new features chapter.

PHP 5.6.0 also introduces changes that affect compatibility:

Array keys won't be overwritten when defining an array as a property of a class via an array literal.
json_decode() is more strict in JSON syntax parsing.
Stream wrappers now verify peer certificates and host names by default when using SSL/TLS.
GMP resources are now objects.
Mcrypt functions now require valid keys and IVs.
For users upgrading from PHP 5.5, a full migration guide is available, detailing the changes between 5.5 and 5.6.0.

For source downloads of PHP 5.6.0, please visit our downloads page. Windows binaries can be found on windows.php.net/download/. The full list of changes is available in the ChangeLog.