Child themes, the template hierarchy and functions.php

The first customisation request on a WordPress job is usually small: change a colour, shorten excerpts, add a banner to one page type. Doing it in the parent theme means the next update erases it. A child theme is the standard way to keep changes, and knowing the template hierarchy tells you which file to override.

Creating a child theme

A child theme is a folder in wp-content/themes with two files. The style.css header must name the parent's folder in Template, matching exactly.

/*
Theme Name: Storefront Child
Template: storefront
Version: 1.0
*/

Then functions.php loads the parent stylesheet followed by the child's own:

<?php
add_action( 'wp_enqueue_scripts', 'storefront_child_styles' );
function storefront_child_styles() {
    wp_enqueue_style( 'parent-style', get_template_directory_uri() . '/style.css' );
    wp_enqueue_style(
        'child-style',
        get_stylesheet_directory_uri() . '/style.css',
        array( 'parent-style' ),
        wp_get_theme()->get( 'Version' )
    );
}

Note the two functions: get_template_directory_uri() always points at the parent, get_stylesheet_directory_uri() at the active (child) theme. Mixing them up is a frequent interview question.

Which template file loads

WordPress walks a list from most specific to least specific and uses the first file it finds, checking the child theme before the parent. Three examples you should be able to recite:

• A page with slug about and ID 12: page-about.php, then page-12.php, then page.php, then singular.php, then index.php.
• A single post: single-post.php, then single.php, then singular.php, then index.php.
• The category news: category-news.php, then category.php, then archive.php, then index.php.

To override a template, copy it from the parent into the child with the same name and edit the copy. Block themes use the same idea with HTML files in a templates folder and a theme.json for styles.

Actions and filters

An action lets you run code at a moment, such as when scripts are enqueued. A filter lets you change a value before WordPress uses it and must return the value. Both go in the child's functions.php, which loads before the parent's, so the parent's functions still run.

add_filter( 'excerpt_length', function ( $length ) {
    return 30;
} );

add_action( 'wp_footer', function () {
    echo '<p class="site-note">Updated weekly.</p>';
} );

Common mistakes and a practice habit

• Copying the parent's whole functions.php into the child, which causes duplicate function errors.
• A Template value that does not match the parent folder name, so the child shows as broken.
• Forgetting to return a value from a filter, which blanks the output.
• Putting business logic such as post types in the theme; it vanishes when the theme changes. Use a plugin.

Practice habit: on a local install, create a child theme, override single.php with one visible change, then switch themes and back to confirm the child survives. Do it twice until it takes under ten minutes.