• Use PHP's shared empty array instance when unserializing empty arrays in php 7.3+.
  (helps slightly with memory usage when repeatedly unserializing,
  when removing elements from arrays before unserializing them,
  or when serializing values including an empty array that was unserialized)
• Emit a deprecation notice when serializing resources.
  PHP itself is converting many resources to objects that throw an Error on serialization attempts.
  Continue to represent resources as null in the serialized data.
• Fix memory management bug when unserializing invalid data (duplicate properties in objects (e.g. from __sleep) or duplicate fields in arrays (impossible for valid data)).
• Speed up calls to __serialize/__unserialize in php 8.0+.
• Fix error messages for unserialize_callback_func: make messages properly refer to the autoload function.
• Optimize unserializing alternative names for private/protected constants that were previously public.