About

An action is a name for an event that is executed at specific points throughout the WordPress core. This name must be a valid array key (string or number).

The event is called by using do_action( 'event-name' );.

Example:

do_action( 'wp_footer' );

Developers can create a custom action using the Action API to add or remove code from an existing action by specifying any existing Hook. This process is called "hooking".

For example: A developer may want to add code to the footer of a theme. This could be accomplished by writing new function, then hooking it to the wp_footer action:

add_action( 'wp_footer', 'my_custom_callback' );

function my_custom_callback()
{
    echo 'Hello world!';
}

The callback function doesn’t have to return anything, because that will be ignored by WordPress. It is sometimes useful to return a value to make unit-tests easier.

• See also the Codex: Filter, Hook, Terminology Confusion
• Related articles: Actions, Action Reference, add_action()
• Forum posts: Filters vs. Actions Discussion and Explanation