Sometimes we experience such error message, which shows up on header area of WordPress site, or generates blank screen.
PHP Warning: Cannot modify header information - headers already sent by (output started at /public_html/.../wp-content/themes/.../functions.php:1) in /.../wp-includes/option.php on line 563
In this case, functions.php file is the cause, it might be others in other cases.
To solve this problem, I just opened functions.php file with Notepad++ ,
- Firstly, make sure there is no space before opening PHP tag <?php or after closing PHP tag ?> (If there is closing tag) , if space exists, eliminate it.
- Secondly, under Encoding, convert it to ANSI
- Save the file.