Director support for PHP

by Miklos Vajna for SWIG

SWIG's directors allow "callback" functionality to be wrapped. Essentially this enables a user to inherit a target language class from a C++ class enabling cross language polymorphism. SWIG currently only supports directors for Python, Ruby, Java, C#, and Ocaml. In particular, PHP support is lacking, but it's certainly possible to implement - and this summer I would like to work on this.