add_action and add_filter in Wordpress

Introduction

These functions let you add your own code to WordPress at specific points. They help you change or add features without changing WordPress core files.

You want to run your code when WordPress loads a page, like adding a message.

You want to change how WordPress shows content, like modifying post titles.

You want to add extra features to plugins or themes without editing their files.

You want to customize WordPress behavior safely during updates.

You want to run code after a user logs in or when a post is saved.

Syntax

Wordpress

add_action('hook_name', 'your_function_name', priority, accepted_args);
add_filter('hook_name', 'your_function_name', priority, accepted_args);

add_action runs your function at a specific event.

add_filter changes data by passing it through your function.

Examples

This adds a message at the bottom of every page.

Wordpress

add_action('wp_footer', 'show_footer_message');
function show_footer_message() {
  echo '<p>Thanks for visiting!</p>';
}

This changes every post title by adding 'Hello:' before it.

Wordpress

add_filter('the_title', 'add_prefix_to_title');
function add_prefix_to_title($title) {
  return 'Hello: ' . $title;
}

This runs your function early when WordPress starts.

Wordpress

add_action('init', 'custom_init_function', 10, 0);
function custom_init_function() {
  // Code to run early in WordPress loading
}

Sample Program

This code adds a blue welcome message centered in the footer of every page. It also adds a star before every post title.

Wordpress

<?php
// Add a message to the footer
add_action('wp_footer', 'footer_message');
function footer_message() {
  echo '<p style="text-align:center; color:blue;">Welcome to my site!</p>';
}

// Change post titles by adding a star
add_filter('the_title', 'star_title');
function star_title($title) {
  return '⭐ ' . $title;
}
?>

Important Notes

Always use unique function names to avoid conflicts.

Priority controls the order your function runs; lower numbers run first.

Filters must return the modified data; actions do not return anything.

Summary

add_action lets you run code at specific WordPress events.

add_filter lets you change data before WordPress uses it.

Use these to customize WordPress safely without changing core files.