W3C DDR Simple API for PHP

PHP port of the W3C DDR Simple api recommendation.

Conversion notes

This set of interfaces is meant to implement the W3C DDR Simple API in PHP. Since the W3C recommendation targets strongly typed languages and PHP is a loosely typed language we made some changes for the PHP interfaces. We also tried to apply PSR-1 and PSR-2 coding standards and PHP best practices.

List of changes

* Suffixed al interfaces with ```Interface```;
* PHP does not support method overloading so overloaded methods in the original interfaces are merged in one method
for this case;