Copy UPGRADING entry last with rebase

Author: Girgias

UPGRADING

@@ -26,6 +26,14 @@ PHP 8.3 UPGRADE NOTES
     (`fiber.stack_size-zend.reserved_stack_size` for fibers).
   . Class constants can now be accessed dynamically using the C::{$name} syntax.
     RFC: https://wiki.php.net/rfc/dynamic_class_constant_fetch
+  . isset() and empty() now check that the offset for string values if of the
+    correct type i.e. numeric, similarly to array and objects.
+    This means code similar to:
+    $a = 'string';
+    var_dump(isset($a['b']));
+    var_dump(empty($a['b']));
+    will now warn with: "Cannot access offset of type string on string"
+    when previously it silently returned false.
 
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 2. New Features