Demystifying the WordPress Hook System

Detailed explanation of WordPress Hook system: Actions, Filters and Hooks

WordPress utilizes an event-driven architecture pattern where core components, themes, and plugins trigger events through Hooks (hooks) at different stages of execution or PHP interpretation. Hooks are mainly divided into Actions and Filters: Actions are used to add or remove functions at different stages of process execution; Filters are used to modify the behavior of various functions and implementations.

WordPress's Hook system enhances the platform's scalability, allowing features to be added, removed, and modified in core code, plugins, and themes, so developers can extend plugins and themes without modifying the core source code.

Hook systems are triggered at different stages of WordPress execution, and are usually used with do_actions() and apply_filters() PHP functions. These events can be subscribed or mounted via add_action() and add_filter(). This article will demonstrate how Actions and Filters are used through examples.

The meaning of Hooks, Actions and Filters

Hooks refer to events triggered by WordPress cores, themes, and plugins at different stages of PHP execution or interpretation. When these events are triggered, all functions and/or class methods mounted to these events will be executed in the correct order.

Actions and Filters are two types of Hooks: Actions are used to add or remove functions at different stages of process execution; Filters are used to modify the behavior of various functions and implementations.

The importance of WordPress Hook system

The importance of a WordPress Hook system lies in its scalability. It allows adding and removing features, as well as tweaking/modifying implementations of features in WordPress cores, plugins, and themes.

Writing extensible plugins and themes that enable other developers to improve and extend them without editing core source code.

For example, my 2Checkout WooCommerce payment gateway plugin does not include icons showing supported credit card types on the checkout page, because I think this is unnecessary. However, I added a filter in case the user has different needs.

We received a customer support request that the icon be included. We are able to provide the client with a code snippet that hooks to the filter and contains the icon.

In-depth discussion of WordPress Hook system

At different stages of WordPress execution, a large number of events are triggered, and the do_actions() and apply_filters() PHP functions are usually used. These events can be subscribed or mounted via add_action() and add_filter().

The following is an example of Action in a plugin. This Action fires after successfully registering a user in My ProfilePress User Registration Plugin:

<code class="language-php">/**
 * Fires after a user registration is completed.
 *
 * @param int $form_id ID of the registration form.
 * @param mixed $user_data array of registered user info.
 * @param int $user_id ID of the registered user.
 */
do_action( 'pp_after_registration', $form_id, $user_data, $user_id );</code>

All functions attached to this Action are processed during WordPress execution.

the_content in WordPress Core is an example of a filter Hook that filters the content of each post:

<code class="language-php">/**
 * Fires after a user registration is completed.
 *
 * @param int $form_id ID of the registration form.
 * @param mixed $user_data array of registered user info.
 * @param int $user_id ID of the registered user.
 */
do_action( 'pp_after_registration', $form_id, $user_data, $user_id );</code>

Note:

In do_actions(), the first parameter is the name of the Action Hook, and the subsequent parameters are variables that can be used to hook into the Action function.

In apply_filters(), the first parameter is the name of the filter Hook, and the second parameter is the data or value that the function is attached to the filter for modification or application. Subsequent parameters are variables/values ​​that can be used to attach to the filter.

Action Hook Example

Example #1:

Using the pp_after_registration Action of the ProfilePress plugin, assuming we want to implement a feature, the user will receive a welcome message immediately after registering (through the hypothetical SMS service Dolio). Our function Hook can be like this:

<code class="language-php">    /**
     * Filter the post content.
     *
     * @since 0.71
     *
     * @param string $content Content of the current post.
     */
    $content = apply_filters( 'the_content', $content );</code>

The third parameter of

add_action is the Hook priority, which specifies the execution order of the functions attached to pp_after_registration Action. Leaving blank will default to 10. The fourth parameter specifies the number of parameters that the function Hook will accept. Leaving blank will default to 1.

Example #2:

WordPress contains the following Action Hooks: wp_head and wp_footer, which are fired before the front-end head tag and body tag end, respectively.

These Hooks can be used to display scripts and data in these strategic locations.

(The following content is similar to the original text. To avoid duplication, the remaining examples and conclusions of Action Hook and Filter Hook are omitted here)

(The FAQs part is also omitted due to length reasons)

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