diff --git a/php/ext/google/protobuf/protobuf.h b/php/ext/google/protobuf/protobuf.h
index c456c194b944b..2a71c5f328c42 100644
--- a/php/ext/google/protobuf/protobuf.h
+++ b/php/ext/google/protobuf/protobuf.h
@@ -2,6 +2,11 @@
 // Copyright 2008 Google Inc.  All rights reserved.
 // https://developers.google.com/protocol-buffers/
 //
+// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+// ! THIS FILE ONLY APPROACHING IN-TREE PHP EXTENSION BUILD !
+// ! DOES NOT USE NORMALLY.                                 !
+// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+//
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met: