Include external files in functions.php WordPress

You can include external files in functions.php by using either

require_once();

or else you can use

include_once();

The difference between these two is that in the event that the file is not found require will give out a fatal error and then stop executing the rest of the file where as include will only issue a warning but will continue the execution of the file.

Yes you can include the external file in functions.php

require_once( get_template_directory() . 'anyfilename.php' );

If you are working with a Child Theme but the path of your include route is not found:

Try the following:

include_once( get_stylesheet_directory() .'/xxxx/anyfilename.php');

Check get_stylesheet_directory() to see your child theme path.