[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.c

@@ -0,0 +1,13 @@
+/* File : example.c */
+
+#include "example.h"
+
+double dot_product(Vector a, Vector b) {
+  return (a.x*b.x + a.y*b.y + a.z*b.z);
+}
+
+void vector_add(Vector a, Vector b, Vector* result) {
+  result->x = a.x + b.x;
+  result->y = a.y + b.y;
+  result->z = a.z + b.z;
+}