Add additional functions file instead of functions.php

You can include multiple files in your functions.php. I do this to keep things organized. All my functions are in a functions subfolder, so my functions.php file just looks like this:

require get_template_directory() . '/functions/function-1.php';
require get_template_directory() . '/functions/function-2.php';
require get_template_directory() . '/functions/function-3.php';

If you have functions you only want called on certain pages, for example, you can do this:

if ( is_page( 'registration-form' ) ) :
    require get_template_directory() . '/functions/registration.php';
endif;