The authorization process is just to ask the user permission to access the API on his behalf.
Once the authorization is obtained, you get an access token to access the API when the user is not present.
That access token is stored in the class access_token variable.
By default the class also saves access tokens in session variables. But sessions only exist in Web accesses from the same user.
If you call the class from the command line, the user is not present, so it cannot access the user session data.
So, you need to copy the value of the access_token variable to somewhere (a database for instance) and restore it when you need it to access the API offline.
Alternatively, the class comes with a subclass that can store authorization tokens in a database, and another subclass to store it specifically in a MySQL database using the mysqli extension.
You can read more about this in this article about off-line access. It uses Google API as example, but it works the same with the Box API.
phpclasses.org/blog/package/7700/po
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