[php] Generate PHP type declarations

We now automatically generate PHP type declarations for PHP >= 8.0.

The generated code still compiles with PHP 7.x but without type declarations.

Examples/test-suite/li_factory.i

@@ -16,7 +16,8 @@
   struct Geometry {
     enum GeomType{
       POINT,
-      CIRCLE
+      CIRCLE,
+      SHAPELESS
     };
 
     virtual ~Geometry() {}