There is a very interesting comment on the php website regarding headers and UTF8 encoded files, which deals with the persistent “headers already sent” problem php might have with such files. One can read that here:

http://in2.php.net/manual/en/function.header.php#79579

If you have recently upgraded your wordpress and you are getting the following errors:
Warning: array_key_exists() [function.array-key-exists]: The first argument should be either a string or an integer in /wp-includes/category-template.php on line 176

The simplest way is to open the page “category-template.php” in your wp-includes directory, go down to line 176, which should be
if(array_key_exists($category, $categories))
to
if(array_key_exists('$category', $categories))

This should solve all the problems.