How to specify the path for require_once in a child theme?

Since 4.7 get_theme_file_path() is the right function to use:

require_once get_theme_file_path( 'gateway/config.php' );

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)