how can solve this pdf problem

hi as im currently working with conversion of PHP file to PDF file. i followed http://www.fpdf.org/ this site. iam getting error like this ?> FPDF-Merge Error: Some data ha开发者_运维技巧s already been output, can't send PDF file

how can i resolve this problem.if my sound is not clear sorry. please visit that page http://www.fpdf.org/

You must not print anything before requesting FPDF to send the document.

FPDF needs to set some HTTP headers to tell the client about the nature of the response (a PDF). However if your script already printed anything the HTTP headers have been sent and FPDF can't set it's own.