Cannot modify header information

Check that <?php is at the very start of the functions.php file (before any whitespace)
and remove ?> at the end of that file.

header or setcookie or anything else that sends HTTP headers has

 to be done before 

any other output, or you'll get that warning/error.

Remember... anything not inluded in <?php ... ?> 

Use a better text editor to remove what's invisible now before the opening <?php.