What Is The functions.php File in WordPress?

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The functions.php file, or theme functions file, is a template included in WordPress themes. The functions.php file behaves like a WordPress plugin, adding functionality and features to a WordPress site. The file automatically activates with the current theme and uses PHP code to change default features or add new ones.

Editing the functions.php file with custom codes allows you to enhance your site with taxonomies, shortcodes, post types, and more. For instance, a WP theme might add some code to the theme’s functions.php file to add a custom welcome message or a new widget area to the footer of the dashboard.

Functions.php vs WordPress Plugins

There are advantages and compromises to using a functions.php rather than a WordPress plugin

A functions.php file:

  • Requires no unique header text
  • Is stored in the theme’s subdirectory in wp-content/themes
  • Executes only when in the active theme’s directory
  • Applies only to that theme (if the theme is changed, you can no longer use the features) 
  • Can have numerous blocks of code used for many purposes

A WordPress plugin:

  • Requires specific, unique header text
  • Is stored in wp-content/plugins, usually in a subdirectory
  • Only executes on page load when activated
  • Applies to all themes
  • Should have a single purpose – e.g., offer SEO features or help with backups

Take Note: Use functions.php With Caution

If you decide to edit your functions.php file, use utmost caution for the following reasons: 

  • First, edits to the functions file will be lost when the theme is updated.
  • Second, modifications will be lost if you change your WordPress theme.
  • Finally, making coding errors in the functions file can lock you out of your site.

Making a coding mistake, even something as simple as a missing semicolon could make your entire site disappear with the “White Screen of Death” error and other WP errors. Only edit a functions.php file if you have a child theme, and the code snippets will only be used with the active child theme. For example, you may want to include snippets for stylesheets, custom fonts or add a language translation file for that particular child theme


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