Backward Incompatible Changes
Changes made to Windows support
Migrating from PHP 5.2.x to PHP 5.3.x
PHP Manual

## New features

PHP 5.3.0 offers a wide range of new features:

- Support for namespaces has been added.
- Support for Late Static Bindings has been added.
- Support for jump labels (limited goto) has been added.
- Support for native Closures (Lambda/Anonymous functions) has been added.
- There are two new magic methods, __callStatic() and __invoke().
- Nowdoc syntax is now supported, similar to Heredoc syntax, but with single quotes.
- It is now possible to use Heredocs to initialize static variables and class properties/constants.
- Heredocs may now be declared using double quotes, complementing the Nowdoc syntax.
- Constants can now be declared outside a class using the const keyword.
- The ternary operator now has a shorthand form: ?:.
- The HTTP stream wrapper now considers all status codes from 200 to 399 to be successful.
- Dynamic access to static methods is now possible.
- Exceptions can now be nested.
- A garbage collector for circular references has been added, and is enabled by default.
- The mail() function now supports logging of sent email. (Note: This only applies to email sent through this function.)

Backward Incompatible Changes
Changes made to Windows support
Migrating from PHP 5.2.x to PHP 5.3.x
PHP Manual