What has changed in PHP 5.5.x
New features
Migrating from PHP 5.4.x to PHP 5.5.x
PHP Manual

## Backward Incompatible Changes

Although most existing PHP 5 code should work without changes, please take note of some backward incompatible changes:

### Windows XP and 2003 support dropped

Support for Windows XP and 2003 has been dropped. Windows builds of PHP now require Windows Vista or newer.

### Case insensitivity no longer locale specific

All case insensitive matching for function, class and constant names is now performed in a locale independent manner according to ASCII rules. This improves support for languages using the Latin alphabet with unusual collating rules, such as Turkish and Azeri.

This may cause issues for code that uses case insensitive matches for non-ASCII characters in multibyte character sets (including UTF-8), such as accented characters in many European languages. If you have a non-English, non-ASCII code base, then you will need to test that you are not inadvertently relying on this behaviour before deploying PHP 5.5 to production systems.

### PHP logo GUIDs removed

The GUIDs that previously resulted in PHP outputting various logos have been removed. This includes the removal of the functions to return those GUIDs. The removed functions are:

- php_logo_guid()
- php_egg_logo_guid()
- php_real_logo_guid()
- zend_logo_guid()

What has changed in PHP 5.5.x
New features
Migrating from PHP 5.4.x to PHP 5.5.x
PHP Manual