[PHP] Add support for specifying any PHP interfaces a wrapped class

implements, e.g.: %typemap("phpinterfaces") MyIterator "Iterator";

Examples/test-suite/php_iterator.i

@@ -0,0 +1,20 @@
+/* php_iterator.i - PHP-specific testcase for wrapping to a PHP Iterator */
+%module php_iterator
+
+%typemap("phpinterfaces") MyIterator "Iterator";
+
+%inline %{
+
+class MyIterator {
+  int i, from, to;
+public:
+  MyIterator(int from_, int to_)
+    : i(from_), from(from_), to(to_) { }
+  void rewind() { i = from; }
+  bool valid() const { return i != to; }
+  int key() const { return i - from; }
+  int current() const { return i; }
+  void next() { ++i; }
+};
+
+%}