diff --git a/service_container/tags.rst b/service_container/tags.rst
index 6f85af4a945..0cb8974b8d1 100644
--- a/service_container/tags.rst
+++ b/service_container/tags.rst
@@ -113,6 +113,11 @@ If you want to apply tags automatically for your own services, use the
                     ->tag('app.custom_tag');
         };
 
+.. caution::
+
+    If you're using PHP, you need to call ``instanceof`` before any service
+    registration to make sure tags are correctly applied.
+
 It is also possible to use the ``#[AutoconfigureTag]`` attribute directly on the
 base class or interface::