Filter hooks in Wordpress - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a filter hook in WordPress?

A filter hook lets you change data before WordPress uses or shows it. It’s like a checkpoint where you can edit text, options, or content.

beginner

How do you add a filter hook in WordPress?

Use the add_filter() function with the filter name, your function name, priority, and number of arguments.

beginner

What does the callback function in a filter hook do?

It receives data, changes it if needed, and returns the new data back to WordPress.

intermediate

Why use filter hooks instead of changing core WordPress files?

Filter hooks keep your changes safe during updates and make your code easier to manage and share.

intermediate

What happens if a filter callback does not return a value?

WordPress will get no data back, which can break the site or cause unexpected results. Always return the modified or original data.

Which function is used to add a filter hook in WordPress?

A apply_filter()

B add_filter()

C add_action()

D register_filter()

The correct function to add a filter hook is add_filter(). add_action() is for action hooks.

What must a filter callback function always do?

A Return the filtered data

B Print the filtered data

C Modify global variables

D Call add_filter() again

A filter callback must return the filtered data so WordPress can use it.

When is a filter hook applied in WordPress?

A Only in the admin dashboard

B After data is saved

C Only during plugin activation

D Before data is saved or displayed

Filter hooks run before data is saved or shown, allowing you to change it.

What does the priority parameter in add_filter() control?

A Order in which filters run

B Number of arguments passed

C Whether the filter is active

D The filter’s name

Priority controls the order filters run; lower numbers run first.

Can multiple filter hooks be added to the same filter name?

A No, filters must have unique names

B No, only one filter per name

C Yes, they run in priority order

D Yes, but only the last one runs

Multiple filters can attach to the same filter name and run in order by priority.

Explain how filter hooks work in WordPress and why they are useful.

Think about how you can change text or options without editing WordPress files.

Describe the steps to create a custom filter hook in WordPress.

Focus on the function and how it connects to WordPress.

Practice

1. What is the main purpose of a filter hook in WordPress?

easy

A. To add new pages to the WordPress admin menu

B. To modify data before WordPress uses it

C. To create new database tables

D. To send emails automatically

Solution

1. Step 1: Understand what filter hooks do

   Filter hooks allow you to change or modify data before WordPress uses it internally or displays it.

2. Step 2: Compare options to the definition

   Only To modify data before WordPress uses it describes modifying data before WordPress uses it, which matches the purpose of filter hooks.

3. Final Answer:

   To modify data before WordPress uses it -> Option B

4. Quick Check:

   Filter hooks = modify data before use [OK]

Hint: Filter hooks change data before WordPress uses it [OK]

Common Mistakes:

• Confusing filter hooks with action hooks
• Thinking filter hooks add new features instead of modifying data
• Believing filter hooks create database tables

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

easy

A. add_filter('the_title', 'my_title_filter');

B. add_action('the_title', 'my_title_filter');

C. filter_add('the_title', 'my_title_filter');

D. add_filter('the_title', my_title_filter);

Solution

1. Step 1: Identify the correct function name and syntax

   The correct function to add a filter hook is add_filter with the hook name as a string and the callback function name as a string.

2. Step 2: Check each option for syntax correctness

   add_filter('the_title', 'my_title_filter'); uses add_filter with both parameters as strings, which is correct. add_action('the_title', 'my_title_filter'); uses add_action, which is for actions, not filters. filter_add('the_title', 'my_title_filter'); uses a wrong function name. add_filter('the_title', my_title_filter); misses quotes around the callback function name.

3. Final Answer:

   add_filter('the_title', 'my_title_filter'); -> Option A

4. Quick Check:

   Use add_filter with hook and callback as strings [OK]

Hint: Use add_filter('hook', 'function_name') with quotes [OK]

Common Mistakes:

• Using add_action instead of add_filter
• Not quoting the callback function name
• Using incorrect function names like filter_add

3. What will be the output of this code snippet?

function add_exclamation($text) {
  return $text . '!';
}
add_filter('the_title', 'add_exclamation');
$title = apply_filters('the_title', 'Hello');
echo $title;

medium

A. Hello

B. Hello!!

C. Hello!

D. Error: undefined function

Solution

1. Step 1: Understand the filter function and hook

   The function add_exclamation adds an exclamation mark to the text. It is hooked to 'the_title' filter.

2. Step 2: Apply the filter to the string 'Hello'

   Calling apply_filters('the_title', 'Hello') runs the filter function, returning 'Hello!'.

3. Final Answer:

   Hello! -> Option C

4. Quick Check:

   Filter adds '!' to 'Hello' = 'Hello!' [OK]

Hint: apply_filters runs all filter functions on the value [OK]

Common Mistakes:

• Expecting original text without modification
• Thinking filter adds multiple exclamations
• Confusing add_filter with add_action

4. Identify the error in this filter hook code:

function change_content($content) {
  echo strtoupper($content);
}
add_filter('the_content', 'change_content');

medium

A. The function should return the modified content, not echo it

B. The hook name 'the_content' is invalid

C. add_filter should be add_action here

D. The function name cannot be 'change_content'

Solution

1. Step 1: Review the filter function requirements

   Filter functions must return the modified value so WordPress can use it.

2. Step 2: Check the function implementation

   The function uses echo instead of returning the modified content, so WordPress receives no changed value.

3. Final Answer:

   The function should return the modified content, not echo it -> Option A

4. Quick Check:

   Filter functions must return values, not echo [OK]

Hint: Filter functions must return modified data, not print it [OK]

Common Mistakes:

• Using echo instead of return in filter functions
• Confusing filter hooks with action hooks
• Assuming hook names are invalid without checking

5. You want to modify the excerpt text to add 'Read more...' only if the excerpt is not empty. Which filter hook function correctly does this?

function add_read_more($excerpt) {
  // Your code here
}
add_filter('get_the_excerpt', 'add_read_more');

hard

A. echo $excerpt . ' Read more...';

B. return $excerpt . ' Read more...';

C. if (!$excerpt) { return 'Read more...'; } else { return $excerpt; }

D. if ($excerpt) { return $excerpt . ' Read more...'; } else { return $excerpt; }

Solution

1. Step 1: Understand the requirement

   We add ' Read more...' only if the excerpt is not empty (truthy).

2. Step 2: Analyze each option's logic

   if ($excerpt) { return $excerpt . ' Read more...'; } else { return $excerpt; } checks if excerpt is truthy, then appends text and returns it; else returns original excerpt. return $excerpt . ' Read more...'; always appends text even if excerpt is empty. if (!$excerpt) { return 'Read more...'; } else { return $excerpt; } adds 'Read more...' only if excerpt is empty, which is opposite. echo $excerpt . ' Read more...'; uses echo instead of return, which is incorrect for filters.

3. Final Answer:

   if ($excerpt) { return $excerpt . ' Read more...'; } else { return $excerpt; } -> Option D

4. Quick Check:

   Return modified excerpt only if not empty [OK]

Hint: Return modified value only when excerpt is not empty [OK]

Common Mistakes:

• Appending text without checking if excerpt is empty
• Using echo instead of return in filter functions
• Reversing the condition logic