1. Use your FTP program to login to your web server.
  2. Navigate to your file, in the example, it's the wp-config.php.
  3. Open up wp-config.php file.
  4. Look for <?php at the very beginning of the file. Remove any empty space before <?php
  5. The <?php which is the PHP opening tag should be at the very first line of the file within wp-config.php
  6. Look for any ?> sign at the end of the file, it there is any, you will need to remove any empty space after it.
  7. If you cannot find any ?> sign at the end of the file, you will not need to do anything. Just to play safe, you can remove empty lines.
  8. If your error points to functions.php, you can carry out the same procedure.