[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/reference/example.h

@@ -0,0 +1,22 @@
+/* File : example.h */
+
+class Vector {
+private:
+  double x,y,z;
+public:
+  Vector() : x(0), y(0), z(0) { }
+  Vector(double x, double y, double z) : x(x), y(y), z(z) { }
+  friend Vector operator+(const Vector &a, const Vector &b);
+  char *as_string();
+};
+
+class VectorArray {
+private:
+  Vector *items;
+  int     maxsize;
+public:
+  VectorArray(int maxsize);
+  ~VectorArray();
+  Vector &operator[](int);
+  int size();
+};