PHP API for creating access and refresh tokens outside of HTTP flow

Problem/Motivation

I need to programmatically generate access_token & refresh_token for a user. I will know the users id & client_id against which the user has to be authenticated. As done from backend, we wont have and need the users password.

I tried using the class use
League\OAuth2\Server\Grant\AbstractGrant;

And then tried to access the logic for token as below:

   $access_token = $this->issueAccessToken(
      $accessTokenTTL,
      $client,
      $this->getDefaultUser($client),
      $finalized_scopes
    );

After several hours of attempts, I am still stuck. Is there a easy way to do this? In my site, users authenticate via SMS and wont't have a password.