What does "page headers already sent" mean?

"I've just installed Nucleus/a plugin/upgraded/done something to my Nucleus files and Nucleus is giving me this error:

Warning: Cannot add header information - headers already sent by (output started at /home/www/path/to/certain/nucleus/file.php:LINENUMBER) in /home/www/path/to/another/nucleus/php/file.php on line LINENUMBER

What is this all about?"

Open up the file Nucleus is talking about and check that there is no whitespace before the <? at the very beginning of the file and after the ?> parts at the end of the file. So if your file looks like this:

White space in the beginning of the file.

the white space is in the circled part just before the <? part. Just delete it so that your file looks like this:

No white space in the file.

Original forum thread (too many to count - here's the latest one):

Further reading:

section: Troubleshooting | submitted by Leng on 2005.Jun.11 | 10504 views

item rate
Total votes: 7 - Rating: 8.57

Please tell us how useful this answer was to you (0 = useless, 10 = very very helpful):