How to customize via a child theme

If you want to change more than just the stylesheet, your child theme can override any file in the parent theme: simply include a file of the same name in the child theme directory, and it will override the equivalent file in the parent theme directory when your site loads. For instance, if you want to change the PHP code for the site header, you can include a header.php in your child theme’s directory, and that file will be used instead of the parent theme’s header.php.

You can also include files in the child theme that are not included in the parent theme. For instance, you might want to create a more specific template than is found in your parent theme, such as a template for a specific page or category archive. See the Template Hierarchy for more information about how WordPress decides what template to use.

Using functions.php

Unlike style.css, the functions.php of a child theme does not override its counterpart from the parent. Instead, it is loaded in addition to the parent’s functions.php. (Specifically, it is loaded right before the parent’s file.)

In that way, the functions.php of a child theme provides a smart, trouble-free method of modifying the functionality of a parent theme. Say that you want to add a PHP function to your theme. The fastest way would be to open its functions.php file and put the function there. But that’s not smart: The next time your theme is updated, your function will disappear. But there is an alternative way which is the smart way: you can create a child theme, add a functions.php file in it, and add your function to that file. The function will do the exact same job from there too, with the advantage that it will not be affected by future updates of the parent theme. Do not copy the full content of functions.php of the parent theme into functions.php in the child theme.

The structure of functions.php is simple: An opening PHP tag at the top, and below it, your bits of PHP. In it you can put as many or as few functions as you wish. The example below shows an elementary functions.php file that does one simple thing: Adds a favicon link to the head element of HTML pages.

<?php // Opening PHP tag - nothing should be before this, not even whitespace

// Custom Function to Include
function my_favicon_link() {
    echo '<link rel="shortcut icon" type="image/x-icon" href="/favicon.ico" />' . "\n";
}
add_action( 'wp_head', 'my_favicon_link' );

Referencing / Including Files in Your Child Theme

When you need to include files that reside within your child theme’s directory structure, you will use get_stylesheet_directory(). Because the parent template’s style.css is replaced by your child theme’s style.css, and your style.css resides in the root of your child theme’s subdirectory, get_stylesheet_directory() points to your child theme’s directory (not the parent theme’s directory).

Here’s an example, using require_once, that shows how you can use get_stylesheet_directory when referencing a file stored within your child theme’s directory structure.

require_once( get_stylesheet_directory() . '/my_included_file.php' );