Programmatically create roles

I wonder if it is possible to create roles programmatically (= by PHP, I have full access to the database).

Use case: I want to assign a unique role to each user. I need this to be able to serve very specific content to individual users (as well as groups of users).

For example: all managers may view an employees performance review, as well as the employee in question.

If this is possible, is there a maximum number of possible roles. I need a few thousand…

Cheers,

Martijn