WordPress Actions vs Filters: The Complete Guide with Real Examples

// Basic action: add something to the footer
add_action( 'wp_footer', 'my_plugin_footer_script' );

function my_plugin_footer_script() {
    echo '<script>console.log("Plugin loaded");</script>';
}

add_action( string $hook_name, callable $callback, int $priority = 10, int $accepted_args = 1 );

// Runs before most other callbacks on init
add_action( 'init', 'my_early_init', 1 );

// Runs after most other callbacks on init
add_action( 'init', 'my_late_init', 99 );

function my_early_init() {
    // fires first
}

function my_late_init() {
    // fires last
}

// The save_post hook passes 3 arguments: $post_id, $post, $update
add_action( 'save_post', 'my_save_post_handler', 10, 3 );

function my_save_post_handler( int $post_id, WP_Post $post, bool $update ) {
    if ( $update ) {
        // Post is being updated, not created
        error_log( 'Post updated: ' . $post->post_title );
    }
}

// Filter: modify the excerpt length
add_filter( 'excerpt_length', 'my_custom_excerpt_length' );

function my_custom_excerpt_length( int $length ): int {
    return 30; // Return 30 words instead of the default 55
}

add_filter( string $hook_name, callable $callback, int $priority = 10, int $accepted_args = 1 );

// Modify post content before display
add_filter( 'the_content', 'my_append_signature' );

function my_append_signature( string $content ): string {
    if ( ! is_single() ) {
        return $content; // Don't modify on archive pages
    }
    return $content . '<p class="author-sig">Written by Kamal Hosen</p>';
}

// Modify the login error message (hide which field is wrong for security)
add_filter( 'login_errors', 'my_generic_login_error' );

function my_generic_login_error( string $error ): string {
    return 'Invalid credentials. Please try again.';
}

// Change the number of posts per page on a specific post type archive
add_filter( 'pre_get_posts', 'my_portfolio_posts_per_page' );

function my_portfolio_posts_per_page( WP_Query $query ): void {
    if ( ! is_admin() && $query->is_main_query() && $query->is_post_type_archive( 'portfolio' ) ) {
        $query->set( 'posts_per_page', 12 );
    }
    // Note: <a href="https://kamalhosen.com/pre-get-posts-wordpress/">pre_get_posts</a> is an action that receives the query by reference,
    // but it's also used as a filter pattern — no return needed here
}

// Remove a named function hook
remove_action( 'wp_head', 'wp_generator' ); // Remove WP version from head
remove_filter( 'the_content', 'wpautop' );  // Remove auto paragraph wrapping

// The priority must match what was used in add_action/add_filter
// Default priority is 10 — if the original used a different priority, match it:
remove_action( 'woocommerce_before_main_content', 'woocommerce_output_content_wrapper', 10 );

// If a plugin registered a hook like this:
// add_action( 'init', [ $this, 'setup' ] );

// You need the object instance to remove it
global $my_plugin_instance;
remove_action( 'init', [ $my_plugin_instance, 'setup' ] );

// Anonymous functions CANNOT be removed — avoid them for public hooks
// This cannot be undone by other code:
add_action( 'init', function() {
    // anonymous — irremovable
} );

// In your plugin — fire a custom action
function my_plugin_process_order( int $order_id ): void {
    // ... do processing ...

    // Let other code run after order processing
    do_action( 'my_plugin_after_order_processed', $order_id );
}

// In your plugin — apply a custom filter
function my_plugin_get_price( float $base_price, int $product_id ): float {
    // Allow other code to modify the price
    return apply_filters( 'my_plugin_product_price', $base_price, $product_id );
}

// Another plugin/theme can now hook into yours:
add_action( 'my_plugin_after_order_processed', function( int $order_id ) {
    // Send a Slack notification, update a CRM, etc.
} );

add_filter( 'my_plugin_product_price', function( float $price, int $product_id ): float {
    return $price * 0.9; // Apply 10% discount
}, 10, 2 );

class My_Plugin {

    public function __construct() {
        add_action( 'init', [ $this, 'register_post_types' ] );
        add_action( 'wp_enqueue_scripts', [ $this, 'enqueue_assets' ] );
        add_filter( 'the_content', [ $this, 'filter_content' ] );
    }

    public function register_post_types(): void {
        // register_post_type() calls here
    }

    public function enqueue_assets(): void {
        wp_enqueue_style( 'my-plugin', plugin_dir_url( __FILE__ ) . 'assets/style.css' );
    }

    public function filter_content( string $content ): string {
        // modify and return
        return $content;
    }
}

// Instantiate once
new My_Plugin();

add_action( 'init', [ 'My_Plugin', 'static_method' ] );
// or shorthand:
add_action( 'init', 'My_Plugin::static_method' );

// Check if a function is hooked to an action
if ( has_action( 'init', 'my_function' ) ) {
    echo 'my_function is hooked to init';
}

// Get the priority it's registered at
$priority = has_filter( 'the_content', 'my_content_filter' );
// Returns false if not hooked, or the priority integer if it is

// Check what's hooked to a given action (inspect the global)
global $wp_filter;
var_dump( $wp_filter['init'] ); // Shows all callbacks and priorities

// The best approach: install Query Monitor plugin
// It shows all hooks fired on the current request, their callbacks, and priorities