WordPress Theme Development

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 take advantage of in order 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 templatestemplate tags, and the WordPress Loop to generate different website results and looks.
  • To provide alternative templates for specific site features, such ascategory pages and search result pages.
  • To quickly switch between two site layouts, or to take advantage of aTheme 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 residing in wp-content/themes/. 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.

WordPress Themes typically consist of three main types of files, in addition to images and JavaScript files. One is the stylesheet called style.css, which controls the presentation (visual design and layout) of the website pages. The second is the optional functions file (functions.php). The other files are the template files which control the way the site pages generate the information from your WordPress database to be displayed on the site. Let’s look at these individually.

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 Ten”:

/*
Theme Name: Twenty Ten
Theme URI: http://wordpress.org/
Description: The 2010 default theme for WordPress.
Author: the WordPress team
Version: 1.0 
Tags: black, blue, white, two-columns, fixed-width, custom-header, custom-background, threaded-comments, sticky-post, translation-ready, microformats, rtl-language-support, editor-style, custom-menu (optional)

License:
License URI:

General comments (optional).
*/

The comment header lines in style.css are required for WordPress to be able to identify a Theme and display it in theAdministration Panel under DesignThemes 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.

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 or this great tutorial by op111.

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:

  • Define functions used in several template files of your theme.
  • Enable features such as post thumbnails, custom header and background, and navigation menus.
  • Set up an options menu, giving site owners options for colors, styles, and other aspects of your theme.

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.

Template Files

Templates are PHP source files used to generate the pages requested by visitors, and are output as HTML. 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 theTemplate 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 direction is RTL. you can generate it using a plugin called the RTLer.
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, it is only used if you use a static front page.
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-books.phpwould be used for displaying single posts from the custom post type booksindex.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 bycategory.phpauthor.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 any other template files, WordPress will use a default version. For example, if you do not have either a comments.php template file, WordPress will automatically use the wp-comments.php template files using Template Hierarchy. (Note: As of version 3.0, the default files aren’t guaranteed to be present or to be the same as they have been. It’s much safer to supply your own template files.)

Typical template files include:

  • comments.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(); ?>

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

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-inTemplate 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 ManageCategories 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 likeget_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 APIfor 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 comment form’s closing tag (<tt</form></tt>). Example plugin use: display a comment preview. As of WordPress 3.0, you should use the default comment form instead, see comment_form().

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

Untrusted Data

You should escape dynamically generated content in your Theme, especially content that is output in 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 titlealt, and value attributes.

In a few cases there might already be a template tag for common cases where safe output is needed. One such case involves the “title” attribute when used with the_title() for post and page titles. To avoid a security vulnerability, usethe_title_attribute() instead. Here’s an example of correct escaping for the title attribute in a post title link when using translatable text:

<?php echo esc_attr( sprintf( __( 'Permanent Link to %s', 'theme-name' ), the_title_attribute( 'echo=0' ) ) ); ?>

Replace deprecated escape calls with the correct calls: wp_specialcharshtmlspecialchar with esc_htmlclean_url withesc_url, and attribute_escape with esc_attr. See Data_Validation for more.

Translation / i18n Support

To ensure smooth transition for language localization, use the gettext functions for wrapping all translatable text within the template files. This makes it easier for the translation files to hook in and translate the titles into the site’s 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)

  • Use the proper DOCTYPE.
  • The opening <html> tag should include language_attributes().
  • The “content-type” meta element should be placed before everything else, including the title element.
  • Use bloginfo() to fetch the title and description.
  • Use automatic feed links to add feed links.
  • Add a call to wp_head(). Plugins use this action hook to add their own scripts, stylesheets, and other functionality.

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(); ?> <?php bloginfo( 'name' ); ?></title>
<link rel="profile" href="http://gmpg.org/xfn/11" />
<link rel="stylesheet" href="<?php bloginfo( 'stylesheet_url' ); ?>" 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 a 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.

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.
    • Respect date and time format settings unless it’s important to the design. (User settings for date and time format are in Settings > General.)
    • For output based on the user setting, use the_time( get_option( 'date_format' ) ).
  • 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.

Comments (comments.php)

  • Author comment should be highlighted differently.
  • Display gravatars (user avatars) if appropriate.
  • Support threaded comments.
  • Display trackbacks/pingbacks.

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 previous search term. 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 (echo or return the value). 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 should be saved in PNG or JPG format.

Theme Options

Themes can optionally include an options page. For example code, see A Sample WordPress Theme Options Page.

When enabling an options page, use the edit_theme_options capability instead of switch_themes for options panels, unless the user actually should be able to switch Themes to be able to use your options panel. WordPress itself uses theedit_theme_options capability for menus, background, header, widgets, et cetera. See more at Roles and Capabilities andAdding Administration Menus.

A note about network mode and Theme options:

If you are using the edit_themes capability anywhere in your Theme, and the Theme is running on a network-enabled WordPress install (previously WordPress MU), be aware that the edit_themes capability used for accessing Theme options pages will prevent site admins in a network from seeing the options menu. Use edit_theme_options instead.

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, IE6, IE7, IE8, 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.

Quoted from: codex.wordpress.org

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