Where to learn more about PHP Basics, and how to apply that to WordPress.

Hi,

I have a pretty good grasp on WordPress now and I am comfortable setting up a WordPress site with a custom theme, what I am not comfortable with is doing specific things in WordPress, like listing child pages for example - this is only an example, I see many more things in the functions.php file or in templates that I do not really understand.

The code below would list child pages but I don't really understand how the code works.

// List child pages
    function wpb_list_child_pages() { 

        global $post; 
        if ( is_page() && $post->post_parent )
            $childpages = wp_list_pages( 'sort_column=menu_order&title_li=&child_of=' . $post->post_parent . '&echo=0' );
        else
            $childpages = wp_list_pages( 'sort_column=menu_order&title_li=&child_of=' . $post->ID . '&echo=0' );
        if ( $childpages ) {
            $string = '<ul>' . $childpages . '</ul>';
        }
        return $string;
    }
        add_shortcode('wpb_childpages', 'wpb_list_child_pages');

Is there something I should be learning to do with PHP to understand this better or is this WordPress related? I know I need to have an understanding of PHP to get WordPress but I am not sure where to go to learn more about it all .... or if that is what I should even be doing.

Can anyone offer me some advise?

Thanks.