This php error code usually means there is white space in the beginning or end or both parts of the page with. The offending page can be the page that shows the error or a page that it calls from within the code.

For example this code was taken from an error off  www.myimghost.com :

Warning: Cannot modify header information – headers already sent by (output started at /home/myimghos/public_html/upload.php:2) in /home/myimghos/public_html/source/includes/data.php on line 20

Warning: Cannot modify header information – headers already sent by (output started at /home/myimghos/public_html/upload.php:2) in /home/myimghos/public_html/source/includes/data.php on line 21

In this case I checked the page upload.php and there was an extra space at the end the page. I removed the white space but still got the error so I check the other listed page: data.php and there was white space at the top and bottom of that page. So  removed it and Voila!