[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/value/example.h

@@ -0,0 +1,8 @@
+/* File : example.h */
+
+typedef struct {
+     double x, y, z;
+} Vector;
+
+double dot_product(Vector a, Vector b);
+void vector_add(Vector a, Vector b, Vector* result);