ถามหน่อยครับพี่น้อง มันขึ้น error แบบนี้อะครับ Cannot modify header ination - headers already sent by

Select * From month WHERE month2='\'03\'' and Year='\'2553\'' ORDER BY CustomerID ASC
Warning: Cannot modify header information - headers already sent by (output started at C:\AppServ\www\mis4\administater_mis\config\config_Pdf\connect_month_ot.php:15) in C:\AppServ\www\mis4\administater_mis\lib\fpdf.php on line 1022
FPDF error: Some data has already been output to browser, can't send PDF file