Is it possible to use add_filter in an included file in the child theme's functions.php?

Question

I am trying something basic but I can't seem to figure it out or I am not even sure if it is possible to be done in a child theme.

What is am trying is to organize the the codes in the functions.php file in the child theme by creating a separate directory for include files and have all the files in there.

Basically, here is the code in my functions.php file:

require_once( get_stylesheet_directory_uri() . '/inc/myfile.php');

And then myfile.php got this code:

function myfunction( $src ) {

}
add_filter( 'script_loader_src', 'myfunction');

But then I get this error:

PHP Fatal error:  Call to undefined function add_filter()

So now I am wondering if there is a way for me to do what I am trying to achieve in a child theme. I have tried this approach in a parent theme and it seemed to work fine.

Thanks in advance for explanation and suggestions.

Answer 1

What you're trying to do is possible.

When including php files, use get_stylesheet_directory() in child themes and get_template_directory() in parent themes.

E.g. require_once( get_stylesheet_directory() . '/inc/myfile.php');

get_stylesheet_directory_uri() and get_template_directory_uri() will return URLs. Those functions are useful when enqueuing public assets such as stylesheets and JavaScript.

Also, setting allow_url_include to Off in your php.ini will provide better error reporting in these situations (source).