header() problem

[meancity]

when i user the header to lead user to another link i always meet the problem

the error returns as

Warning: Cannot add header information - headers already sent by (output started at c:\program files\apache group\apache\htdocs\blahA.php:13) in c:\program files\apache group\apache\htdocs\blahB.php on line 3020

in this case blahA.php has a line

require("blahB.php");

so what actually is "Cannot add header information - headers already sent"?? and how can i avoid it to happen?

anyone knows , kinldy answer my doubt

appreciate in advance

[laserlight]

it usually implies that output has been sent (i.e. html code or blank lines output) before the header in question is sent