[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.h

@@ -0,0 +1,46 @@
+/* File : example.h */
+
+#include <stdio.h>
+
+class Shape {
+public:
+  Shape() {
+    nshapes++;
+  }
+  virtual ~Shape() {
+    nshapes--;
+  }
+  double  x, y;
+  void    move(double dx, double dy);
+  virtual double area(void) = 0;
+  virtual double perimeter(void) = 0;
+  static  int nshapes;
+  static  int get_nshapes();
+};
+
+class Circle : public Shape {
+private:
+  double radius;
+public:
+  Circle(double r) : radius(r) { }
+  ~Circle() { }
+  virtual double area(void);
+  virtual double perimeter(void);
+};
+
+class Square : public Shape {
+private:
+  double width;
+public:
+  Square(double w) : width(w) { }
+  ~Square() { }
+  virtual double area(void);
+  virtual double perimeter(void);
+};
+
+const char *overloaded( int i );
+const char *overloaded( double d );
+const char *overloaded( const char * str );
+const char *overloaded( const Circle& );
+const char *overloaded( const Shape& );
+