PHP 5.5.0のKey featuresは以下の通り。
Added generators and coroutines.
Added the finally keyword.
Added a simplified password hashing API.
Added support for constant array/string dereferencing.
Added scalar class name resolution via ::class.
Added support for using empty() on the result of function calls and other expressions.
Added support for non-scalar Iterator keys in foreach
Added support for list() constructs in foreach statements.
Added Zend Opcache extension and enable building it by default.
The GD library has been upgraded to version 2.1 adding new functions and improving existing functionality.
A lot more improvements and fixes.