FIND_IN_SET() how to implement in mysql and codeigniter

userid = 1 have this:
$users->category_id = array(1,2,3)
userid = 2 have this:
$users->category_id = array(1,2,3,4,5,6)

So:
$this->db->where_in('category_id',array(1,2,3))->get('category');