session_start() and header('location:') redirect

Possible Duplicate: Headers already sent by PHP I'm having a problem with `session_start()` and `header('Location:')` on a log out php file. I'm linking to the log out file using a simple anchor tag from a html file. The logout php file is this: ``` <?php session_start(); session_destroy(); header("Location: index.php"); ?> ``` To see where the errors where I used, ``` error_reporting(E_ALL); ini_set('display_errors', '1'); ``` I got a warning that "Warning: session_start(): Cannot send session cache limiter - headers already sent" on line 4 (where `session_start();` is) and Warning: Cannot modify header information - headers already sent on line 6 (where `header('Location')` is). I've had a look around for others who have had similar problems. I've made sure there's no whitespace before the `session_start()`. Nothing is being outputted before it. I've tried only setting `session_start()` if it's not already set. I've tried buffering it with `ob_start()`. I tried a relative and absolute path for the redirect just to make sure. But nothing has worked. Am I missing something? Any help is much appreciated.