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Theme Development

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This article is about developing WordPress Themes. If you wish to learn more about how to install and use Themes, review Using Themes. This topic differs from Using Themes because it discusses the technical aspects of writing code to build your own Themes rather than how to activate Themes or where to obtain new Themes.

Why WordPress Themes

WordPress Themes are files that work together to create the design and functionality of a WordPress site. Each Theme may be different, offering many choices for site owners to instantly change their website look.

You may wish to develop WordPress Themes for your own use, for a client project or to submit to the WordPress Theme Directory. Why else should you build a WordPress Theme?

  • To create a unique look for your WordPress site.
  • To take advantage of templates, template tags, and the WordPress Loop to generate different website results and looks.
  • To provide alternative templates for specific site features, such as category pages and search result pages.
  • To quickly switch between two site layouts, or to take advantage of a Theme or style switcher to allow site owners to change the look of your site.

A WordPress Theme has many benefits, too.

  • It separates the presentation styles and template files from the system files so the site will upgrade without drastic changes to the visual presentation of the site.
  • It allows for customization of the site functionality unique to that Theme.
  • It allows for quick changes of the visual design and layout of a WordPress site.
  • It removes the need for a typical WordPress site owner to have to learn CSS, HTML, and PHP in order to have a great-looking website.

Why should you build your own WordPress Theme? That's the real question.

  • It's an opportunity to learn more about CSS, HTML, and PHP.
  • It's an opportunity to put your expertise with CSS, HTML, and PHP to work.
  • It's creative.
  • It's fun (most of the time).
  • If you release it to the public, you can feel good that you shared and gave something back to the WordPress Community (okay, bragging rights)

Theme Development Standards

WordPress Themes should be coded using the following standards:

Anatomy of a Theme

WordPress Themes live in subdirectories of the WordPress themes directory (wp-content/themes/ by default) which can not be directly moved using the wp-config.php file. The Theme's subdirectory holds all of the Theme's stylesheet files, template files, and optional functions file (functions.php), JavaScript files, and images. For example, a Theme named "test" would reside in the directory wp-content/themes/test/. Avoid using numbers for the theme name, as this prevents it from being displayed in the available themes list.

WordPress includes a default theme in each new installation. Examine the files in the default theme carefully to get a better idea of how to build your own Theme files.

For a visual guide, see this infographic on WordPress Theme Anatomy.

WordPress Themes typically consist of three main types of files, in addition to images and JavaScript files.

  1. The stylesheet called style.css, which controls the presentation (visual design and layout) of the website pages.
  2. WordPress template files which control the way the site pages generate the information from your WordPress database to be displayed on the site.
  3. The optional functions file (functions.php) as part of the WordPress Theme files.

Let's look at these individually.

Child Themes

The simplest Theme possible is a child theme which includes only a style.css file, plus any images. This is possible because it is a child of another theme which acts as its parent.

For a detailed guide to child themes, see Child Themes.

Theme Stylesheet

In addition to CSS style information for your theme, style.css provides details about the Theme in the form of comments. The stylesheet must provide details about the Theme in the form of comments. No two Themes are allowed to have the same details listed in their comment headers, as this will lead to problems in the Theme selection dialog. If you make your own Theme by copying an existing one, make sure you change this information first.

The following is an example of the first few lines of the stylesheet, called the stylesheet header, for the Theme "Twenty Thirteen":

/*
Theme Name: Twenty Thirteen
Theme URI: http://wordpress.org/themes/twentythirteen
Author: the WordPress team
Author URI: http://wordpress.org/
Description: The 2013 theme for WordPress takes us back to the blog, featuring a full range of post formats, each displayed beautifully in their own unique way. Design details abound, starting with a vibrant color scheme and matching header images, beautiful typography and icons, and a flexible layout that looks great on any device, big or small.
Version: 1.0
License: GNU General Public License v2 or later
License URI: http://www.gnu.org/licenses/gpl-2.0.html
Tags: black, brown, orange, tan, white, yellow, light, one-column, two-columns, right-sidebar, flexible-width, custom-header, custom-menu, editor-style, featured-images, microformats, post-formats, rtl-language-support, sticky-post, translation-ready
Text Domain: twentythirteen

This theme, like WordPress, is licensed under the GPL.
Use it to make something cool, have fun, and share what you've learned with others.
*/

NB: The name used for the Author is suggested to be the same as the Theme Author's wordpress.org username, although it can be the author's real name as well. The choice is the Theme Author's.

Note the list of Tags used to describe the theme. These allow user to find your theme using the tag filter. Here is the full list of the tags allowed.

The comment header lines in style.css are required for WordPress to be able to identify the Theme and display it in the Administration Panel under Design > Themes as an available Theme option along with any other installed Themes.

Stylesheet Guidelines

  • Follow CSS coding standards when authoring your CSS.
  • Use valid CSS when possible. As an exception, use vendor-specific prefixes to take advantage of CSS3 features.
  • Minimize CSS hacks. The obvious exception is browsers-specific support, usually versions of IE. If possible, separate CSS hacks into separate sections or separate files.
  • All possible HTML elements should be styled by the Theme, both in post/page content and in comment content.
    • Tables, captions, images, lists, block quotes, et cetera.
  • Adding print-friendly styles is highly recommended.
    • You can include a print stylesheet with media="print" or add in a print media block in your main stylesheet.

Functions File

A theme can optionally use a functions file, which resides in the theme subdirectory and is named functions.php. This file basically acts like a plugin, and if it is present in the theme you are using, it is automatically loaded during WordPress initialization (both for admin pages and external pages). Suggested uses for this file:

The default WordPress theme contains a functions.php file that defines many of these features, so you might want to use it as a model. Since functions.php basically functions as a plugin, the Function_Reference list is the best place to go for more information on what you can do with this file.

Note for deciding when to add functions to functions.php or to a specific plugin: You may find that you need the same function to be available to more than one parent theme. If that is the case, the function should be created in a plugin instead of a functions.php for the specific theme. This can include template tags and other specific functions. Functions contained in plugins will be seen by all themes.

Template Files

Templates are PHP source files used to generate the pages requested by visitors, and are output as HTML. Template files are made up of HTML, PHP, and WordPress Template Tags.

Let's look at the various templates that can be defined as part of a Theme.

WordPress allows you to define separate templates for the various aspects of your site. It is not essential, however, to have all these different template files for your site to fully function. Templates are chosen and generated based upon the Template Hierarchy, depending upon what templates are available in a particular Theme.

As a Theme developer, you can choose the amount of customization you want to implement using templates. For example, as an extreme case, you can use only one template file, called index.php as the template for all pages generated and displayed by the site. A more common use is to have different template files generate different results, to allow maximum customization.

Template Files List

Here is the list of the Theme files recognized by WordPress. Of course, your Theme can contain any other stylesheets, images, or files. Just keep in mind that the following have special meaning to WordPress -- see Template Hierarchy for more information.

style.css
The main stylesheet. This must be included with your Theme, and it must contain the information header for your Theme.
rtl.css
The rtl stylesheet. This will be included automatically if the website's text direction is right-to-left. This can be generated using the the RTLer plugin.
index.php
The main template. If your Theme provides its own templates, index.php must be present.
comments.php
The comments template.
front-page.php
The front page template.
home.php
The home page template, which is the front page by default. If you use a static front page this is the template for the page with the latest posts.
single.php
The single post template. Used when a single post is queried. For this and all other query templates, index.php is used if the query template is not present.
single-{post-type}.php
The single post template used when a single post from a custom post type is queried. For example, single-book.php would be used for displaying single posts from the custom post type named "book". index.php is used if the query template for the custom post type is not present.
page.php
The page template. Used when an individual Page is queried.
category.php
The category template. Used when a category is queried.
tag.php
The tag template. Used when a tag is queried.
taxonomy.php
The term template. Used when a term in a custom taxonomy is queried.
author.php
The author template. Used when an author is queried.
date.php
The date/time template. Used when a date or time is queried. Year, month, day, hour, minute, second.
archive.php
The archive template. Used when a category, author, or date is queried. Note that this template will be overridden by category.php, author.php, and date.php for their respective query types.
search.php
The search results template. Used when a search is performed.
attachment.php
Attachment template. Used when viewing a single attachment.
image.php
Image attachment template. Used when viewing a single image attachment. If not present, attachment.php will be used.
404.php
The 404 Not Found template. Used when WordPress cannot find a post or page that matches the query.

These files have a special meaning with regard to WordPress because they are used as a replacement for index.php, when available, according to the Template Hierarchy, and when the corresponding Conditional Tag returns true. For example, if only a single post is being displayed, the is_single() function returns 'true', and, if there is a single.php file in the active Theme, that template is used to generate the page.

Basic Templates

At the very minimum, a WordPress Theme consists of two files:

  • style.css
  • index.php

Both of these files go into the Theme directory. The index.php template file is very flexible. It can be used to include all references to the header, sidebar, footer, content, categories, archives, search, error, and any other page created in WordPress.

Or, it can be divided into modular template files, each one taking on part of the workload. If you do not provide other template files, WordPress may have default files or functions to perform their jobs. For example, if you do not provide a searchform.php template file, WordPress has a default function to display the search form.

Typical template files include:

  • comments.php
  • comments-popup.php
  • footer.php
  • header.php
  • sidebar.php

Using these template files you can put template tags within the index.php master file to include these other files where you want them to appear in the final generated page.

Here is an example of the include usage:

<?php get_sidebar(); ?>

<?php get_footer(); ?>

The default files for some template functions may be deprecated or not present, and you should provide these files in your theme. As of version 3.0, the deprecated default files are located in wp-includes/theme-compat. For example, you should provide header.php for the function get_header() to work safely, and comments.php for the function comments_template().

For more on how these various Templates work and how to generate different information within them, read the Templates documentation.

Custom Page Templates

The files defining each Page Template are found in your Themes directory. To create a new Custom Page Template for a Page you must create a file. Let's call our first Page Template for our Page snarfer.php. At the top of the snarfer.php file, put the following:

<?php
/*
Template Name: Snarfer
*/
?>

The above code defines this snarfer.php file as the "Snarfer" Template. Naturally, "Snarfer" may be replaced with most any text to change the name of the Page Template. This Template Name will appear in the Theme Editor as the link to edit this file.

The file may be named almost anything with a .php extension (see reserved Theme filenames for filenames you should not use; these are special file names WordPress reserves for specific purposes).

What follows the above five lines of code is up to you. The rest of the code you write will control how Pages that use the Snarfer Page Template will display. See Template Tags for a description of the various WordPress Template functions you can use for this purpose. You may find it more convenient to copy some other Template (perhaps page.php or index.php) to snarfer.php and then add the above five lines of code to the beginning of the file. That way, you will only have to alter the HTML and PHP code, instead of creating it all from scratch. Examples are shown below. Once you have created the Page Template and placed it in your Theme's directory, it will be available as a choice when you create or edit a Page. (Note: when creating or editing a Page, the Page Template option does not appear unless there is at least one template defined in the above manner.)

Query-based Template Files

WordPress can load different Templates for different query types. There are two ways to do this: as part of the built-in Template Hierarchy, and through the use of Conditional Tags within The Loop of a template file.

To use the Template Hierarchy, you basically need to provide special-purpose Template files, which will automatically be used to override index.php. For instance, if your Theme provides a template called category.php and a category is being queried, category.php will be loaded instead of index.php. If category.php is not present, index.php is used as usual.

You can get even more specific in the Template Hierarchy by providing a file called, for instance, category-6.php -- this file will be used rather than category.php when generating the page for the category whose ID number is 6. (You can find category ID numbers in Manage > Categories if you are logged in as the site administrator in WordPress version 2.3 and below. In WordPress 2.5 the ID column was removed from the Admin panels. You can locate the category id by clicking 'Edit Category' and looking on the URL address bar for the cat_ID value. It will look '...categories.php?action=edit&cat_ID=3' where '3' is the category id). For a more detailed look at how this process works, see Category Templates.

If your Theme needs to have even more control over which Template files are used than what is provided in the Template Hierarchy, you can use Conditional Tags. The Conditional Tag basically checks to see if some particular condition is true, within the WordPress Loop, and then you can load a particular template, or put some particular text on the screen, based on that condition.

For example, to generate a distinctive stylesheet in a post only found within a specific category, the code might look like this:

<?php
if ( is_category( '9' ) ) {
    get_template_part( 'single2' ); // looking for posts in category with ID of '9'
} else {
    get_template_part( 'single1' ); // put this on every other category post
}
?>

Or, using a query, it might look like this:

<?php
$post = $wp_query->post;
if ( in_category( '9' ) ) {
    get_template_part( 'single2' );
} else {
    get_template_part( 'single1' );
}
?>

In either case, this example code will cause different templates to be used depending on the category of the particular post being displayed. Query conditions are not limited to categories, however, see the Conditional Tags article to look at all the options.

Defining Custom Templates

It is possible to use the WordPress plugin system to define additional templates that are shown based on your own custom criteria. This advanced feature can be accomplished using the "template_redirect" action hook. More information about creating plugins can be found in the Plugin API reference.

Including Template Files

To load another template (other than header, sidebar, footer, which have predefined included commands like get_header()) into a template, you can use get_template_part(). This makes it easy for a Theme to reuse sections of code.

Referencing Files From a Template

When referencing other files within the same Theme, avoid hard-coded URIs and file paths. Instead reference the URIs and file paths with bloginfo(): see Referencing Files From a Template.

Note that URIs that are used in the stylesheet are relative to the stylesheet, not the page that references the stylesheet. For example, if you include an images/ directory in your Theme, you need only specify this relative directory in the CSS, like so:

h1 {
    background-image: url(images/my-background.jpg);
}

Plugin API Hooks

When developing Themes, it's good to keep in mind that your Theme should be set up so that it can work well with any WordPress plugins users might decide to install. Plugins add functionality to WordPress via "Action Hooks" (see Plugin API for more information).

Most Action Hooks are within the core PHP code of WordPress, so your Theme does not have to have any special tags for them to work. But a few Action Hooks do need to be present in your Theme, in order for Plugins to display information directly in your header, footer, sidebar, or in the page body. Here is a list of the special Action Hook Template Tags you need to include:

wp_head()
Goes in the <head> element of a theme, in header.php. Example plugin use: add JavaScript code.
wp_footer()
Goes in footer.php, just before the closing </body> tag. Example plugin use: insert PHP code that needs to run after everything else, at the bottom of the footer. Very commonly used to insert web statistics code, such as Google Analytics.
wp_meta()
Typically goes in the <li>Meta</li> section of a Theme's menu or sidebar; sidebar.php template. Example plugin use: include a rotating advertisement or a tag cloud.
comment_form()
Goes in comments.php directly before the file's closing tag (</div>). Example plugin use: display a comment preview.

For a real world usage example, you'll find these plugin hooks included in the default Theme's templates.

Theme Customization API

As of WordPress 3.4, a new Theme Customization feature is available by default for nearly all WordPress themes. The Theme Customization admin page is automatically populated with options that a theme declares support for with add_theme_support() or using the Settings API, and allows admins to see non-permanent previews of changes they make in real time.

Theme and plugin developers interested in adding new options to a theme's Theme Customization page should see the documentation on the Theme Customization API. Additional tutorials on the Theme Customization API are available at the Ottopress.com website.

Untrusted Data

You should escape dynamically generated content in your Theme, especially content that is output to HTML attributes. As noted in WordPress Coding Standards, text that goes into attributes should be run through esc_attr() so that single or double quotes do not end the attribute value and invalidate the XHTML and cause a security issue. Common places to check are title, alt, and value attributes.

There are few special template tags for common cases where safe output is needed. One such case involves outputing a post title to a title attribute using the_title_attribute() instead of the_title() to avoid a security vulnerability. Here's an example of correct escaping for the title attribute of a post title link when using translatable text:

<a href="<?php the_permalink(); ?>" title="<?php sprintf( __( 'Permanent Link to %s', 'theme-name' ), the_title_attribute( 'echo=0' ) ); ?>"><?php the_title(); ?></a>

Replace deprecated escape calls with the correct calls: wp_specialchars() and htmlspecialchars() with esc_html(), clean_url() with esc_url(), and attribute_escape() with esc_attr(). See Data_Validation for more.

Translation Support / I18n

To ensure smooth transition for language localization, use the WordPress gettext-based i18n functions for wrapping all translatable text within the template files. This makes it easier for the translation files to hook in and translate the labels, titles and other template text into the site's current language. See more at WordPress Localization and I18n for WordPress Developers.

Theme Classes

Implement the following template tags to add WordPress-generated class attributes to body, post, and comment elements. For post classes, apply only to elements within The Loop.

Template File Checklist

When developing a Theme, check your template files against the following template file standards.

Document Head (header.php)

Here's an example of a correctly-formatted HTML5 compliant head area:

<!DOCTYPE html>
<html <?php language_attributes(); ?>>
    <head>
        <meta charset="<?php bloginfo( 'charset' ); ?>" />
        <title><?php wp_title(); ?></title>
        <link rel="profile" href="http://gmpg.org/xfn/11" />
        <link rel="stylesheet" href="<?php echo get_stylesheet_uri(); ?>" type="text/css" media="screen" />
        <link rel="pingback" href="<?php bloginfo( 'pingback_url' ); ?>" />
        <?php if ( is_singular() && get_option( 'thread_comments' ) ) wp_enqueue_script( 'comment-reply' ); ?>
        <?php wp_head(); ?>
    </head>

Navigation Menus (header.php)

  • The Theme's main navigation should support a custom menu with wp_nav_menu().
    • Menus should support long link titles and a large amount of list items. These items should not break the design or layout.
    • Submenu items should display correctly. If possible, support drop-down menu styles for submenu items. Drop-downs allowing showing menu depth instead of just showing the top level.

Widgets (sidebar.php)

  • The Theme should be widgetized as fully as possible. Any area in the layout that works like a widget (tag cloud, blogroll, list of categories) or could accept widgets (sidebar) should allow widgets.
  • Content that appears in widgetized areas by default (hard-coded into the sidebar, for example) should disappear when widgets are enabled from Appearance > Widgets.

Footer (footer.php)

  • Use the wp_footer() call, to appear just before closing body tag.
<?php wp_footer(); ?>
</body>
</html>

Index (index.php)

  • Display a list of posts in excerpt or full-length form. Choose one or the other as appropriate.
  • Include wp_link_pages() to support navigation links within posts.

Archive (archive.php)

  • Display archive title (tag, category, date-based, or author archives).
  • Display a list of posts in excerpt or full-length form. Choose one or the other as appropriate.
  • Include wp_link_pages() to support navigation links within posts.

Pages (page.php)

  • Display page title and page content.
  • Display comment list and comment form (unless comments are off).
  • Include wp_link_pages() to support navigation links within a page.
  • Metadata such as tags, categories, date and author should not be displayed.
  • Display an "Edit" link for logged-in users with edit permissions.

Single Post (single.php)

  • Include wp_link_pages() to support navigation links within a post.
  • Display post title and post content.
    • The title should be plain text instead of a link pointing to itself.
  • Display the post date.
  • Display the author name (if appropriate).
  • Display post categories and post tags.
  • Display an "Edit" link for logged-in users with edit permissions.
  • Display comment list and comment form.
  • Show navigation links to next and previous post using previous_post_link() and next_post_link().

Comments (comments.php)

  • Author comment should be highlighted differently.
  • Display gravatars (user avatars) if appropriate.
  • Support threaded comments.
  • Display trackbacks/pingbacks.
  • This file shouldn’t contain function definitions unless in the function_exist() check to avoid redeclaration errors. Ideally all functions should be in functions.php.

Search Results (search.php)

  • Display a list of posts in excerpt or full-length form. Choose one or the other as appropriate.
  • The search results page show the search term which generated the results. It's a simple but useful way to remind someone what they just searched for -- especially in the case of zero results. Use the_search_query() or get_search_query() (display or return the value, respectively). For example:
    <h2><?php printf( __( 'Search Results for: %s' ), '<span>' . get_search_query() . '</span>'); ?></h2>
  • It's a good practice to include the search form again on the results page. Include it with: get_search_form().

JavaScript

  • JavaScript code should be placed in external files whenever possible.
  • Use wp_enqueue_script() to load your scripts.
  • JavaScript loaded directly into HTML documents (template files) should be CDATA encoded to prevent errors in older browsers.
<script type="text/javascript">
/* <![CDATA[ */
// content of your Javascript goes here
/* ]]> */
</script>

Screenshot

Create a screenshot for your theme. The screenshot should be named screenshot.png, and should be placed in the top level directory. The screenshot should accurately show the theme design and saved in PNG format. The recommended image size is 880x660. The screenshot will only be shown as 387x290, but the over double-sized image allows for high-resolution viewing on HiDPI displays.

Theme Options

Themes can optionally support the Theme Customize Screen. For an example code, see A Sample WordPress Theme Options Page.

When enabling the availability of the Theme Customize Screen for a user role, use the "edit_theme_options" user capability instead of the "switch_themes" capability unless the user role actually should also be able to switch the themes. See more at Roles and Capabilities and Adding Administration Menus.

If you are using the "edit_themes" capability anywhere in your Theme to gain the Adminstrator role access to the Theme Customize Screen (or maybe some custom screens), be aware that since Version 3.0, this capability is not assigned to the Adminstrator role by default in the case of WordPress Multisite installation. See the explanation. In such a case, use the "edit_theme_options" capability instead if you want the Adminstrator to see the "Theme Options" menu. See the additional capabilities of Adminstrator role when using WordPress Multisite.

Theme Testing Process

  1. Fix PHP and WordPress errors. Add the following debug setting to your wp-config.php file to see deprecated function calls and other WordPress-related errors: define('WP_DEBUG', true);. See Deprecated Functions Hook for more information.
  2. Check template files against Template File Checklist (see above).
  3. Do a run-through using the Theme Unit Test.
  4. Validate HTML and CSS. See Validating a Website.
  5. Check for JavaScript errors.
  6. Test in all your target browsers. For example, IE7, IE8, IE9, Safari, Chrome, Opera, and Firefox.
  7. Clean up any extraneous comments, debug settings, or TODO items.
  8. See Theme Review if you are publicly releasing the Theme by submitting it to the Themes Directory.

Resources and References

Code Standards

Theme Design

CSS

Templates

Functions listing

Testing and QA

Release & Promotion

External Resources & Tutorials