Cannot modify header information – headers already sent by (output started at /h

Dear all,

currently I am trying to reload my page after the entry in a form with the command:

header("Location: " . $_SERVER['REQUEST_URI']);
exit();

Having searched on this forum and on stackoverflow I did not find a final solution.

-> How did you manage to use the twenty thirteen Theme and do some form processing with php and refreshing the page?

I am getting the error: Cannot modify header information – headers already sent by (output started at …/example.com/wordpress/wp-content/themes/twentythirteen/inc/custom-header.php:122)

I noticed two things:
-> The file custom-header.php contains LOTS of <?php sections. Is it safe to remove all but the one at the beginning?
To the theme authors: What is the reason behind that many <?php sections? Where it possible to have only one section?
-> I copied the file to …twentythirteen-Child/inc, but it is not picked up. Where do I need to put the overriding file?

Thank you very much in advance,
MZZH