Manual:$wgSessionProviders/en

Details
Specifies what providers to use for. By default,  (for normal cookie-based authentication) and   (for bot passwords) are included.

Order and array keys are not relevant. By convention, the array key is the classname of the provider.

Example
or better, in PHP 5.5 and newer:

or, if the extension uses extension registration (preferable):

This will create a session provider object with, and register it as one of the session providers. See for details of the object construction syntax.