Backward Incompatible Changes
Changes in SAPI modules
Migrating from PHP 5.3.x to PHP 5.4.x
PHP Manual

## New features

PHP 5.4.0 offers a wide range of new features:

- Support for traits has been added.
- Short array syntax has been added, e.g. $a = [1, 2, 3, 4]; or $a = ['one' => 1, 'two' => 2, 'three' => 3, 'four' => 4];.
- Function array dereferencing has been added, e.g. foo()[0].
- Closures now support $this.
- <?= is now always available, regardless of the short_open_tag php.ini option.
- Class member access on instantiation has been added, e.g. (new Foo)->bar().
- Class::{expr}() syntax is now supported.
- Binary number format has been added, e.g. 0b001001101.
- Improved parse error messages and improved incompatible arguments warnings.
- The session extension can now track the upload progress of files.
- Built-in development web server in CLI mode.

Backward Incompatible Changes
Changes in SAPI modules
Migrating from PHP 5.3.x to PHP 5.4.x
PHP Manual