Or in a select statement

PHP Code:

function select($table_name, $where_data){
            $this->db->select('*');
          $this->db->from($table_name);
          $this->db->where($where_data);
        $this->db->order_by('id','DESC');
          $query = $this->db->get();
        // echo $str = $this->db->last_query();
          return $query->result();
} 


If I want to do an "or" is this right


$user_data=$this->action->select("table", array("a"=>$a, "b"=>$b) or array("c"=>$c, "b"=>$b));



Thanks