Added UPGRADING note.

Author: dstogov

UPGRADING

@@ -211,6 +211,15 @@ PHP 8.1 UPGRADE NOTES
 5. Changed Functions
 ========================================
 
+- Core:
+  . Properties order used in foreach, var_dump(), serialize(), object comparison
+    etc. was changed. Now properties are naturally ordered according to their
+    declaration and inheritance. Prpoerties declared in a base class are going
+    to be before the child properties. This order is consistent with internal
+    layout of properies in zend_objct structure and repeats the order in
+    default_properties_table[] and properties_info_table[]. The old order was
+    not documented and was caused by class inheritance implementation details.
+
 - Filter:
   . The FILTER_FLAG_ALLOW_OCTAL flag of the FILTER_VALIDATE_INT filter now accept
     octal string with the leading octal prefix ("0o"/"0O")