[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/enum/example.h

@@ -0,0 +1,13 @@
+/* File : example.h */
+
+enum color { RED, BLUE, GREEN };
+
+class Foo {
+ public:
+  Foo() { }
+  enum speed { IMPULSE, WARP, LUDICROUS };
+  void enum_test(speed s);
+};
+
+void enum_test(color c, Foo::speed s);
+