PHP

Call to undefined method CI_DB_mysqli_result::order_by()

In CodeIgniter's Active Record, every method returns the object itself (which allows method chaining) except for get and get_where which return the result set.

Thus, what you need to do is:

$query = $this->db->order_by('birth_date', 'ASC')->get_where($this->tbl_name, $where);