add_action and add_filter in Wordpress - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the add_action function do in WordPress?

add_action lets you run your custom code at specific points during WordPress execution. It "hooks" your function to an event.

beginner

What is the purpose of add_filter in WordPress?

add_filter lets you modify data before WordPress uses or shows it. It "filters" content by running your function on it.

intermediate

How do add_action and add_filter differ in their use?

add_action runs code at certain events without changing data. add_filter runs code that changes or returns data.

intermediate

What parameters does add_action accept?

<p><code>add_action</code> takes: the hook name (string), your function name (callable), priority (number, default 10), and accepted args count (default 1).</p>

intermediate

Why is priority important in add_action and add_filter?

Priority decides the order your function runs if many functions hook to the same event. Lower numbers run first.

What does add_filter primarily do in WordPress?

A Runs code to change data before output

B Runs code only once on page load

C Adds a new admin menu

D Deletes a post automatically

add_filter modifies data by running your function on it before WordPress uses or shows it.

Which parameter is NOT part of add_action?

A Hook name

B Function name

C Database connection

D Priority

add_action does not take a database connection parameter.

If you want your function to run after others on the same hook, what should you do?

A Use a higher priority number

B Use a lower priority number

C Use <code>remove_action</code>

D Change the function name

Higher priority numbers run later, so your function runs after others.

Which hook type lets you add extra HTML to a page without changing data?

A Database hook

B Action hook

C Filter hook

D Security hook

Action hooks let you run code at points in WordPress, like adding HTML output.

What must your function do when used with add_filter?

A Call <code>exit()</code>

B Echo the data directly

C Delete the data

D Return the modified data

Filter functions must return the data, usually modified, so WordPress can use it.

Explain how add_action and add_filter work in WordPress and when to use each.

Think about actions as events and filters as data changes.

Describe the role of priority in add_action and add_filter and how it affects function execution order.

Imagine a queue where priority decides who goes first.

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