All objects in PHP 5 are passed by reference. Constants, can now be defined within the scope of a class. Class methods and properties can now be declared as public, private or protected. Classes and methods can also be declared as final to prevent further inheritance. Interfaces can be used to define common APIs. Abstract classes provide models for class implementations that follow a specific blueprint.PHP 5 - New extensions
PHP Data Objects extension provides a unified DB access extension. Standard PHP Library (SPL) provides numerous interfaces. Reflection Allows for runtime introspection of executing PHP code.