function update($table, $values = array(), $where){
                $sql  = "UPDATE $table SET ";
                $i = 0;
                foreach($values as $row => $value){
                    ++$i;
                    if($i < count($values)){
                        $sql .= $row.' = '.$value.', ';
                    }else{
                        $sql .= $row.' = '.$value;
                    }
                }
                $sql .= " WHERE $where";
                return $sql;
                //$this->query($sql);
        }