WordPress themes give WordPress users the ability to completely change the look of a WP website, as well as add functionality to it. In this three-part series, we’ll introduce WordPress themes, showing how they work, how they’re structured, the PHP architecture behind them, and other relevant information. We’ll then embark on a journey to build a WordPress theme.

This first article prepares us for this journey by discussing the theory behind WordPress themes.

A Word or Two on the Basics

WordPress was conceived as a blog engine, or a simple, blogging-oriented content management system. It was initially released in 2003, by Matt Mullenweg and Mike Little. Ever since then, its user base hasn’t stopped growing. WordPress is a PHP-powered web application that uses MySQL as its database, and is usually run behind a server program, such as NGINX or Apache.

WordPress is basically just a bunch of PHP files that work together as an application. A PHP interpreter uses these files to produce web pages to web visitors. It produces HTML, to be more precise.

The role of the templating engine of WordPress is to enable us to write (primarily) presentational instructions — instructions on how exactly to structure and style the HTML content that WordPress will output. These instructions are encapsulated in WordPress themes.

Each theme consist of a folder with PHP, CSS, and sometimes JavaScript files. The files that every WordPress theme must have — at the minimum — are style.css and index.php . This is the technical minimum required for the theme to function, but no serious WordPress theme stays with only these two files.

Basic template and Partials Files

The minimum index.php file catches all the queries that don’t have their respective specialized template files within a theme. front-page.php , home.php , page.php , taxonomy.php , author.php , archive.php are some of the templates that we can use in our themes to further structure specific pages or queries in our theme.

For example, the archive.php file will specify the HTML output structure when a visitor requests some of the pages that display list of posts. page.php will specify how to display individual pages, and so on.

Partials files encapsulate repeatable parts of pages in WordPress website. For example, the header and footer are usually consistent across all pages on a website, so WordPress themes separate these page parts into header.php and footer.php . comments.php will be used to display comments wherever applicable.

These files are then required from the template files that we explained.

This way, we adhere to the DRY principle, and don’t repeat the code in all those files.

Template Hierarchy

In the WordPress templating system, there’s a hierarchy of template files that WordPress will try to use for each request. This hierarchy is based on specificity. WordPress will try to use the most specific file for each request, if it exists. If it doesn’t exist, it will look up the next, less specific file, and so on.

To explain this on a page request — when the visitor visits a specific page on a WordPress website — WordPress will first try to find the template the page author assigned to it in the wp-admin backend. That can be a completely custom template file, even completely static.

If there’s no such template, or it wasn’t assigned, WordPress will try to find a template file with a slug of that particular page in its filename. That would look like page-mypageslug.php — whatever our mypageslug may be.

The next file WordPress will try to use will be a file with that particular page’s ID in its filename — like page-48.php .

If none of those page-specific files exist, WordPress will try to use page.php — used for all the pages, unless otherwise specified.

If it doesn’t find page.php , it will try to use singular.php . This template file is used when — for posts — single.php is not found, and for pages when page.php is not found.

Now, if none of these are found in our page request example, WordPress will fall back to index.php .

This, briefly, explains the WordPress template hierarchy. All of these template files we mentioned will usually incorporate (require) partials like header.php , footer.php and others as needed. They can also specify their specific partials to use — for example, a page-specific header.

The next thing you need to be familiar with to understand theming is the WordPress post type.

WordPress Post Types

The content in WordPress exists in the form of post types. The built-in types are posts and pages. These are the logical ones. WordPress also has a built-in attachment post type, navigation menus and revisions. These are also, technically, post types.

We can also specify our own post types, whether in our themes or our plugins. We need use the following:

register_post_type ( $post_type , $args )

Here, we specify the post type name, how it’s structured, how it’s represented in wp-admin , and so on.

When we have this defined, we can also create template files specific for this post type. Custom post types, like pages and posts, have their own template hierarchy.

More about the template hierarchy can be found here.

style.css

style.css is a crucial file in every WordPress theme, and its function goes beyond styling. This file is used to provide basic theme information to WordPress. Without this, WordPress would not be able to register a theme as a theme.

The WordPress Codex provides more information about all the details of this file. For the sake of brevity, we’ll review just some of them. In this CSS file’s header comments, we provide WordPress information on the following:

theme name

author

description

theme URI

version

license

and other details

WordPress uses these details to display the theme properly in the back end.

The meta header in the Twenty Seventeen theme, for example, looks like this:

This said, style.css file is, obviously, used to style our WordPress pages.

WordPress Action and Filter Hooks

WordPress — and WordPress themes and plugins — heavily rely on action hooks and filter hooks. These are part of an event-riven architecture. A simple explanation of this would be that, in the process of execution of the page connected to visitors’ web request, there are certain registered points — hooks — that enable us to fire actions at those points. We hook PHP functions to those points to be executed. Those are action hooks. Filter hooks are focused on processing — changing — pieces of data within the execution cycle. So, if the filter hook is registered for any piece of data (PHP variable), we can hook our own function and change, or process, this piece of data.

Although unorthodox, compared to the omnipresent MVC software pattern, the reliance on the hook system in WordPress has played a big role in the popularization of WordPress, as it has made it very simple to plug new functionalities into it, without the need to touch — or even to deeply understand — the core codebase itself.

Hooks are explained here at greater length, and we’ll delve into it more in other parts of this guide.

The Loop

The Loop is the elementary part of the WordPress request cycle. As the WordPress Codex puts it:

The Loop is PHP code used by WordPress to display posts. Using The Loop, WordPress processes each post to be displayed on the current page, and formats it according to how it matches specified criteria within The Loop tags.

The WordPress loop is, basically, a PHP while loop that displays posts according to visitors’ request, and specified criteria in the template file in which it’s specified.

Within the loop, whether it outputs just a single post or a page, or a list of them, we can output various parts of the post, like title and content. We can specify any HTML output or structure we want. We can also use conditional tags, and so on.

The most rudimentary example of the loop might look something like this:

if ( have_posts ( ) ) { while ( have_posts ( ) ) { the_post ( ) ; } }

We can use multiple loops on one page (in which case we need to reset them), and there are specific WordPress functions that exist for use inside it.

WordPress Conditional Tags are snippets of PHP code that enable us to display pieces of content only when certain conditions are satisfied. For example, we have the site-wide header.php , which is used on every page of the website, but we may want certain HTML snippets to be displayed only on the home page or some other page.

So, in the first example, we would use is_front_page() , and it would look something like this:

if ( is_front_page ( ) ) { }

There are lots of conditional tags provided, and they allow for big flexibility in WordPress development.

These are the building blocks of WordPress, but there are also taxonomies — which can be built in or custom ones we can specify — and terms within these taxonomies. We use them to categorize and sort our posts.

Theme Structure

At the end of this part, and before we endeavor to create our own theme, we’ll use the WP-CLI tool to scaffold a minimal theme based on Underscores — maintained by Automattic — to get a glance of the typical, minimal WordPress theme structure:

From this, we can get a sense of the way we can structure our theme files, folders, etc.

Conclusion

In this first part of our guide on WordPress theming, we introduced the terminology and the building blocks of WordPress themes, which we’ll use in the following tutorials to build our own WordPress theme.

There are three articles in this series on building a WordPress theme from scratch: