It's almost New Year, and time for presents (according to the Russian traditions).

I hereby present you the API versioning framework. Navigate to https://gerrit.wikimedia.org/r/#/c/41014/ and get that patch to your local installation for a test run.

There shouldn't be any visible functionality changes. At least that was the goal. But now we can easily add a new version module or a submodule (prop/list/etc) without breaking existing code.

To see it in action -- in ApiMain.php, add this line into $Modules array:
  'query2' => 'ApiQuery',

it adds another version of query implemented by the same existing class.

now if you look at http://localhost/api.php you will see

* action=query *
  This module is obsolete. See old documentation at api.php?action=help&modules=query
* action=query2 *
... -- the regular query parameters -- ...

both query and query2 will work identically in this example unless you change ApiQuery.php.