php: Fix overloaded directed methods with non-void return

We were treating  such methods like constructors and assigning to the
internal _cPtr, which just seems bizarrely wrong.

Fixes #1900

Examples/test-suite/director_overload.i

@@ -47,5 +47,14 @@ public:
   virtual void notover(int *p) const {}
 };
 
-%}
+class OverloadedGetSet
+{
+  int v;
+public:
+  OverloadedGetSet() : v(42) { }
+  virtual ~OverloadedGetSet() { }
+  virtual int rw() const { return v; }
+  virtual void rw(int new_v) { v = new_v; }
+};
 
+%}