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

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

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.
 

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: https://wordpress.org/
Description: The 2010 default theme for WordPress.
Author: wordpressdotorg
Author URI: https://wordpress.org/
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).
*/

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.

The comment header lines in style.css are required for WordPress to be able to identify a 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:

• Enable Theme Features such as Sidebars, Navigation Menus, Post Thumbnails, Post Formats, Custom Headers, Custom Backgrounds and others.
• Define functions used in several template files of your theme.
• 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.

Note For Deciding When To Add Functions To Functions.PHP or 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 .

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, 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-.php

The single post template used when a single post from a custom post type is queried. For example, single-books.php would be used for displaying single posts from the custom post type books. 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.

• To include the header, use get_header().
• To include the sidebar, use get_sidebar().
• To include the footer, use get_footer().
• To include the search form, use get_search_form().

Here is an example of the include usage:

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:

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 .

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. 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.

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:

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

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.

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():.

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" .

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 element of a theme, in header.php. Example plugin use: add JavaScript code.

wp_footer()

Goes in footer.php, just before the closing 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

Meta

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 (