[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/overloading/example.cxx

@@ -0,0 +1,55 @@
+/* File : example.c */
+
+#include "example.h"
+#include <math.h>
+#ifndef M_PI
+#  define M_PI 3.14159265358979323846
+#endif
+
+int Shape::get_nshapes() {
+  return nshapes;
+}
+
+/* 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(void) {
+  return M_PI*radius*radius;
+}
+
+double Circle::perimeter(void) {
+  return 2*M_PI*radius;
+}
+
+double Square::area(void) {
+  return width*width;
+}
+
+double Square::perimeter(void) {
+  return 4*width;
+}
+
+const char *overloaded(int i) {
+  return "Overloaded with int";
+}
+
+const char *overloaded(double d) {
+  return "Overloaded with double";
+}
+
+const char *overloaded(const char * str) {
+  return "Overloaded with char *";
+}
+
+const char *overloaded( const Circle& ) {
+  return "Overloaded with Circle";
+}
+
+const char *overloaded( const Shape& ) {
+  return "Overloaded with Shape";
+}