PHP 5.6 brings with it new and improved features including an interactive debugger, and has some backward incompatible changes.
The headline new feature in PHP 5.6 is constant scalar expressions.
This means it is now possible to provide a scalar expression involving numeric and string literals and/or constants in contexts where PHP previously expected a static value, such as constant and property declarations and default function arguments as demonstrated with this example:
Which produces this output:
Other new features are:
Variadic functions and argument unpacking using the ... operator
Exponentiation using the ** operator
Function and constant importing with the use keyword.
php://input is now reusable, and $HTTP_RAW_POST_DATA is deprecated
GMP (arbitrary length integer) objects now support operator overloading
File uploads larger than 2 gigabytes in size are now accepted
The following changes 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
All these changes are documented in a migration guide and other improvements and bug fixes are listed in the Change Log.
There is also a Getting Started guide for phpdbg, the new interactive debugger which is implemented and distributed as a SAPI module, just as the CLI interface. It integrates with an initializes the Zend environment.