All php classes have been renamed and moved to separate files.
Class autoloading can now be done in accord with the PSR-4 standard.
All global variables and global functions have been removed.
+ Iterating over xmlrpc value objects is now easier thank to support for ArrayAccess and Traversable interfaces.
Backward compatibility is maintained via lib/xmlrpc.inc, lib/xmlrpcs.inc and lib/xmlrpc_wrappers.inc.
For more details, head on to doc/api_changes_v4.md