[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/class/example.cxx

@@ -0,0 +1,28 @@
+/* File : example.cxx */
+
+#include "example.h"
+#define M_PI 3.14159265358979323846
+
+/* Move the shape to a new location */
+void Shape::move(double dx, double dy) {
+  x += dx;
+  y += dy;
+}
+
+int Shape::nshapes = 0;
+
+double Circle::area() {
+  return M_PI*radius*radius;
+}
+
+double Circle::perimeter() {
+  return 2*M_PI*radius;
+}
+
+double Square::area() {
+  return width*width;
+}
+
+double Square::perimeter() {
+  return 4*width;
+}