Write an api to a simple site (yii2)

there is a very simple admin on yii2
it is required to write to it api and data exchange, so that in the future sites that will be synchronized with it can exchange data about users, operations.

there must be an identification of the connected resource, a key (required to be added in the admin), and commands that allow you to write to the DB.

after completion of the work, write a small documentation on how to use, how to connect to other sites. briefly on the case, so that you can give it to other developers.
all sites will be located on remote servers.

Payment upon completion. I would like to do it in a week.