[Trigunflame]

headers sent by header() are not HTML headers, they are HTTP headers which are sent in the "Header" of the HTTP reply back to the client.. ie.

your browser sends something like

GET / HTTP/1.1
Host: vbulletin.org
Connection: keep-alive

Server sends something back like

HTTP/1.1 200 OK
Connection: Keep-Alive
Content-Length: 5
Content-Type: text/plain

Jebus

So if you are getting that error, it means you have already outputted some sort of data, either intentionally or unintentionally..

Rule of thumb is, always output headers with header() before you ever output anything.