Error message!!

Hello I'm using the php3. Before the programs were worked fine, but i have problems since last night. I don't what is problem. I'll send the warning message. If you know what is a matter, would you teach me? Warning: Cannot add more header information - the header was already sent (header information may be added only before any output is generated from the script - check for text or whitespace outside PHP tags, or calls to functions that output text) in "$urladdress/checkloginuser.php3" on line 5 -- Insook Moon Phone: 305-261-7845 email: insook@pasate.com insook_2@yahoo.com