[PHP] Add support for PHP7.

PHP5's C extension API has changed substantially so you need to use
-php7 to specify you want PHP7 compatible wrappers.
Fixes https://github.com/swig/swig/issues/571

Examples/php5/callback/example.i

@@ -0,0 +1,11 @@
+/* File : example.i */
+%module(directors="1") example
+%{
+#include "example.h"
+%}
+
+/* turn on director wrapping Callback */
+%feature("director") Callback;
+
+%include "example.h"
+