diff --git a/UPGRADE.md b/UPGRADE.md
index 6cb2c4db8ff..d0a0dadd564 100644
--- a/UPGRADE.md
+++ b/UPGRADE.md
@@ -8,6 +8,10 @@ awareness about deprecated code.
 
 # Upgrade to 3.4
 
+## Marked `Connection::ARRAY_PARAM_OFFSET` as internal.
+
+The `Connection::ARRAY_PARAM_OFFSET` constant has been marked as internal. It will be removed in 4.0.
+
 ## Deprecated using NULL as prepared statement parameter type.
 
 Omit the type or use `Parameter::STRING` instead.
diff --git a/src/Connection.php b/src/Connection.php
index fe0fd69c723..a85acbca446 100644
--- a/src/Connection.php
+++ b/src/Connection.php
@@ -65,6 +65,8 @@ class Connection
 
     /**
      * Offset by which PARAM_* constants are detected as arrays of the param type.
+     *
+     * @internal Should be used only within the wrapper layer.
      */
     public const ARRAY_PARAM_OFFSET = 100;