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.

Practice

1. What is the main purpose of add_action in WordPress?

easy

A. To create new user roles

B. To change the content of posts before display

C. To register new database tables

D. To run custom code at specific WordPress events

Solution

1. Step 1: Understand what add_action does

   add_action hooks your custom function to WordPress events so it runs at the right time.

2. Step 2: Compare with other options

   Changing content before display is for filters, not actions. Registering tables or roles is unrelated to add_action.

3. Final Answer:

   To run custom code at specific WordPress events -> Option D

4. Quick Check:

   add_action runs code on events = D [OK]

Hint: Actions run code at events; filters change data [OK]

Common Mistakes:

• Confusing add_action with add_filter
• Thinking add_action changes data directly
• Assuming add_action registers database or roles

2. Which of the following is the correct syntax to add a filter in WordPress?

easy

A. add_filter(my_function, 'the_content');

B. add_action('the_content', 'my_function');

C. add_filter('the_content', 'my_function');

D. add_action(my_function, 'the_content');

Solution

1. Step 1: Recall the correct order of parameters for add_filter

   The first parameter is the filter hook name as a string, the second is the callback function name.

2. Step 2: Check each option's parameter order and function used

   add_filter('the_content', 'my_function'); uses add_filter with correct parameter order. Options B and D use add_action incorrectly. add_filter(my_function, 'the_content'); reverses parameters.

3. Final Answer:

   add_filter('the_content', 'my_function'); -> Option C

4. Quick Check:

   Filter syntax: add_filter(hook, function) = A [OK]

Hint: Filter syntax: hook name first, function second [OK]

Common Mistakes:

• Swapping parameters order
• Using add_action instead of add_filter
• Passing function name before hook name

3. Given this code snippet:

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

What will be the output of the_title() for a post titled "World"?

medium

A. Hello: World

B. the_title

C. World

D. Hello World

Solution

1. Step 1: Understand the filter function

   The function change_title adds "Hello: " before the original title.

2. Step 2: Apply the filter to the post title

   When the_title() runs, WordPress applies the filter, so "World" becomes "Hello: World".

3. Final Answer:

   Hello: World -> Option A

4. Quick Check:

   Filter prepends 'Hello: ' to title = C [OK]

Hint: Filters modify data output; here title gets prefix [OK]

Common Mistakes:

• Ignoring the filter effect on output
• Confusing add_action with add_filter effect
• Missing the colon and space after Hello

4. What is wrong with this code?

add_filter('the_content', 'modify_content');
function modify_content() {
  return 'Modified content';
}

medium

A. The hook name is incorrect

B. The callback function must accept the content parameter

C. add_filter should be add_action here

D. The function must echo content, not return

Solution

1. Step 1: Check the callback function signature

   Filter callbacks must accept the original data as a parameter to modify it.

2. Step 2: Identify the missing parameter

   The function modify_content has no parameters, so it cannot receive the content to change.

3. Final Answer:

   The callback function must accept the content parameter -> Option B

4. Quick Check:

   Filter callbacks need input parameter = B [OK]

Hint: Filter functions must accept and return data [OK]

Common Mistakes:

• Omitting the parameter in filter callback
• Using add_action instead of add_filter
• Trying to echo instead of return in filter

5. You want to add a custom message after every post content but only for logged-in users. Which is the best way to do this using add_filter?

hard

A. Use add_filter('the_content', 'add_message'); and inside add_message check if user is logged in before appending message

B. Use add_action('the_content', 'add_message'); and append message without checking user

C. Use add_filter('the_content', 'add_message'); and always append message without user check

D. Use add_action('wp_footer', 'add_message'); to add message after content

Solution

1. Step 1: Choose the correct hook and method

   To modify post content, add_filter('the_content', ...) is correct because it changes content before display.

2. Step 2: Add user check inside callback

   To show message only to logged-in users, check user status inside the callback function before appending the message.

3. Final Answer:

   Use add_filter('the_content', 'add_message'); and inside add_message check if user is logged in before appending message -> Option A

4. Quick Check:

   Filter content + user check = A [OK]

Hint: Filter content and check user inside function [OK]

Common Mistakes:

• Using add_action instead of add_filter for content change
• Not checking user login status
• Adding message in footer instead of content