NEW EDIT TO THIS TOPIC:

what usually results in these errors is poor script design.  when one processes a form, they should do it BEFORE OUTPUTTING ANYTHING TO THE BROWSER.  there is NO reason that the process cannot be operated in the header of the document, before any HTML is output.

let's take a common example.  the programmer wants a login form which checks the username and password against the database.  if they don't match, then don't set a cookie remembering them and tell them it failed.  if they do match, set the cookie and send them to a member page.  many will do:

[HTML starting the page and layout]
[PHP processing the form]
[form code]

because they can simply echo any errors straight from PHP right above that form.  they can also send a success message, set the cookie, and header() them off on their merry way without seeing the form again.  it's a logical place to put this code.  think again.  you'll (perhaps not so) obviously get header errors when you go to use setcookie() and header().  if the login fails, it's dandy, but if you have a successful login the user gets nowhere.

answer:  put the processing in the header, and store the results in variables.  perhaps a $result variable that is 1 if successful, 0 if failed.  then $output that contains either a success message or customized error messages.  the new code would look like:

[PHP processing the form (if it was sent) and storing the results]
[HTML starting the page and layout]
[PHP echoing the results]
[form code if failed - exit(); if successful]

this will solve your header errors and will make for much more maintainable and readable code.  if you want to redirect them elegantly (with a success message), use a <meta> refresh redirect.  header() should only be used in instances where instant redirection is desired.