[php] Use -prefix setting in type declarations

Examples/test-suite/prefix.i

@@ -15,6 +15,16 @@ public:
 // __set and __isset methods weren't getting the prefix.
 class Bar : public Foo {
 public:
+  Bar *get_self() {
+    return this;
+  }
 };
 
+// This failed in git pre 4.1.0 with PHP 8.x because we weren't adding the
+// prefix to class names in type declarations.  Error was at extension load
+// time:
+//
+// Fatal error: Bar must be registered before ProjectBar in Unknown on line 0
+Bar* self(Bar* bar) { return bar->get_self(); }
+
 %}