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WordPress Hooks and Filters

  • has_action(): Checks if any action has been registered to a hook.
  • add_action(): Hooks a function onto a specific action.
  • do_action(): Drops a bookmark in the code for later action hooking.
  • remove_action(): Removes an action from a specific hook.
  • has_filter(): Checks if any filters have been registered for a hook.
  • add_filter(): Hooks into a specific filter.
  • apply_filters(): Calls all functions registered on a particular hook.

WordPress Basics​

  • WordPress is open-source software built on PHP and MySQL.
  • Fully-featured content management system.
  • Two versions: Self-hosted (download, complete control) and Hosted (wordpress.com).
  • Developers interact via Themes (visual layer) and Plugins (additional functionality).

Plugin API Overview​

  • Plugins extend WordPress functionality without modifying the base code.
  • A plugin is a set of PHP functions adding specific features or services.
  • Plugin API consists of hooks and filter hooks.
  • Hooks latch onto WordPress at specific points during runtime.

Action Hooks​

  • Actions and filters are two kinds of hooks.
  • Common action functions: has_action(), add_action(), do_action(), remove_action().
  • Actions run during typical page requests.

Filter Hooks​

  • Filters modify input and return it.
  • Similar to action functions.
  • Basic filter functions: has_filter(), add_filter(), apply_filters().

Priorities, Arguments, and Customization​

  • Priorities specify the order of action function execution.
  • Lower numbers execute earlier.
  • Functions with the same priority execute in order of addition.
  • Arguments are data passed to a function. Default is 1.

Customizing the WordPress Login Page​

  • Plugins should be written within the WordPress installation.
  • Include a header on custom plugins.
  • Basic info needed: Plugin Name, Version, Author, License, Text Domain.
  • Plugins need activation within WordPress.

Adding a Custom Stylesheet​

  • wp-login.php powers the WordPress login page.
  • do_action( 'login_enqueue_scripts' ) sets login page styles.
  • Care needed to avoid function name conflicts.
  • wp_enqueue_style() registers the style source.

Filtering Login Error Messages​

  • Login error message confirms valid username.
  • login-errors hook supplies built-in error messages.
  • Add a function as a filter to change the message.
  • __return_null returns a null value when used with a filter.

Removing the Login Page Shake​

  • Login page shakes on incorrect credentials.
  • login_head hook controls shaking behavior.
  • add_action( 'login_head', 'wp_shake_js', 12 ) triggers the shake.
  • To remove, wrap in another function and call using add_action().

WordPress Hooks, Filters, and Load Order​

  • Navigating documentation is powerful.
  • Documentation covers plugin creation basics, naming conventions, and translation readiness.
  • Plugin API has documentation on hooks and filters.
  • Debug bar plugin provides detailed page load information.

Load Order​

  • Actions are listed in order of execution.
  • Inject code at the appropriate point.
  • rarst.net/wordpress/wordpress-core-load/ displays load order charts.
  • query monitor plugin offers detailed loading info for debugging.

Understanding Callback Functions​

  • Callback functions are passed as arguments.
  • add_action() uses a callback function as the second argument.
  • pre_get_post action provides access to the $query object.
  • Documentation states when it's too late to hook an action.

Using Apply Filters​

  • apply_filters() calls functions added to the filter hook.
  • Takes three parameters: $tag, $value, and $var.
  • Example: $value = apply_filters( 'example_filter', 'filter_me', $arg1, $arg2 ).
  • array_merge() merges two arrays.

WP_Hook vs $wp_filter​

  • WordPress hooks and callback functions are stored in $wp_filter.
  • WP_Hook class resolves recursive callback issues.
  • Direct access to WP_Hook or $wp_filter is usually unnecessary.

Adding Custom Hooks​

  • Create hooks in plugins for interaction.
  • Use apply_filters() and do_action() functions.
  • Be intentional; changing hooks can break compatibility.

Inside Themes and Plugins with Hooks​

  • Popular themes and plugins come with their hooks.
  • Documentation may vary for less popular ones.
  • Some documentation may be locked behind logins.
  • GitHub may have additional information.

Tips for Using Third-Party Hooks​

  • Display an error if plugins are missing dependencies.
  • Codebase of plugins in wordpress.org is accessible.
  • Name plugin files after their folder structure.
  • Exit if plugin file is accessed directly.
  • Check for parent plugin existence.

Building a Demo Plugin​

  • For security, kill plugin if accessed directly.
  • register_sidebar() builds a sidebar definition.
  • dynamic_sidebar() displays a sidebar.
  • wp_enqueue_scripts styles the front page.
  • WordPress conditionals manage display.
  • Provide option to disable style choices.
  • Stay updated with WordPress development at makewordpress.org.