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

I have a child theme folder called themes/child-theme and inside I have a file dashboard_payments.php.
Under the child theme folder I’m creating a new folder called gateway and inside there’s a config.php.

So, how do I do a require_once inside dashboard_payments.php to call the file gateway/config.php? How would the require_once or include line look like?

Answers:

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Method 1

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

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

Method 2

You can use either

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

or (should be faster)

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

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