PHP warning: headers already sent in Unknown [duplicate]

Question

I'm looking for things that might trigger the following PHP warning:

PHP Warning: Cannot modify header information - headers already sent in Unknown on line 0

Answer 1

Turned out that it was the line

ob_start("ob_gzhandler");

that caused the warning. This has been reported and fixed in 2001, it seems, but for some reason it keeps coming back.

Answer 2

It might be a lot of things, but as the others said, it's often just a space lying around somewhere that gets outputted and then a header() command is sent which normally is fine, but not after starting to send content back (potentially just a space in this case).

Using ob_start() stops the output from going out right away by buffering it. So it's a potential solution, or at least way to diagnose where it's coming from as zodeus said.

---

One common thing that causes those lose spaces are in this scenario.

global.php

<?php
  $variable = 1;
  $database = 'something else';
?> <-- A space here
 <-- Or here

index.php

<?php

  require('global.php');
  $var = dosomething();
  header('Location: http://www.example.com/');

?>

One way to fix that is to remove the ?> at the end of the global.php file. You don't need those, they are only useful if you start putting HTML for example after your PHP code. So you'd have:

<?php
  $variable = 1;
  $database = 'something else';

And when you do the require(), the space is not outputted before the header().

---

Just to illustrate the problems with content outputted and headers is that other common case that gives a similar error. It happens when you forget to stop the processing after a redirection using header().

if ($notLoggedIn) {
  header('Location: http://www.example.com/login.php');
}
echo 'Welcome to my website'; // this will be outputted, 
                              // you should have an exit() 
                              // right after the header()