add_action and add_filter in Wordpress - Mini Project: Build & Apply

Using add_action and add_filter in WordPress

📖 Scenario: You are building a simple WordPress plugin to customize how your site works. You want to add a message to the footer and change the title of posts before they show on the site.

🎯 Goal: Build a WordPress plugin that uses add_action to add a footer message and add_filter to modify post titles.

📋 What You'll Learn

Create a function called custom_footer_message that echoes a footer message

Use add_action to hook custom_footer_message to the wp_footer action

Create a function called modify_post_title that adds ' - Custom' to the post title

Use add_filter to hook modify_post_title to the the_title filter

💡 Why This Matters

🌍 Real World

WordPress developers use add_action and add_filter to customize themes and plugins safely and flexibly.

💼 Career

Understanding hooks is essential for WordPress plugin and theme development jobs.

1

Create the footer message function

Create a function called custom_footer_message that echoes the text 'Thank you for visiting!'.

Wordpress

<?php
// Plugin main file

// Your code here

Hint

Use the function keyword to create custom_footer_message. Inside, use echo to show the message.

2

Hook the footer message function to wp_footer

Use add_action to hook the custom_footer_message function to the wp_footer action.

Wordpress

<?php
// Plugin main file

function custom_footer_message() {
    echo 'Thank you for visiting!';
}

// Your code here

Hint

Call add_action with the first argument as 'wp_footer' and the second as 'custom_footer_message'.

3

Create the post title modification function

Create a function called modify_post_title that takes one parameter $title and returns the title with ' - Custom' added at the end.

Wordpress

<?php
// Plugin main file

function custom_footer_message() {
    echo 'Thank you for visiting!';
}

add_action('wp_footer', 'custom_footer_message');

// Your code here

Hint

Define modify_post_title with $title as input. Use return to add ' - Custom' to the title.

4

Hook the post title modification function to the_title filter

Use add_filter to hook the modify_post_title function to the the_title filter.

Wordpress

<?php
// Plugin main file

function custom_footer_message() {
    echo 'Thank you for visiting!';
}

add_action('wp_footer', 'custom_footer_message');

function modify_post_title($title) {
    return $title . ' - Custom';
}

// Your code here

Hint

Call add_filter with 'the_title' as the first argument and 'modify_post_title' as the second.

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