Assigning a post to some categories.

Hi WordPress’s Guru !

I need help. I have lots of posts (300+) which are not assigned to any category. Also I have categories being inputed/set.

How can I assign one post to some categories (1 or more) by my PHP script, not by hand in admin).

I go through all posts via LOOP is there some variant to assign current post in loop to some category ?

Thanks