Filter hooks in Wordpress - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this filter hook modification?

Consider the following WordPress filter hook code. What will be the final output of apply_filters('greet', 'Hello')?

Wordpress

<?php
add_filter('greet', function($text) {
    return $text . ', friend!';
});
add_filter('greet', function($text) {
    return strtoupper($text);
});

echo apply_filters('greet', 'Hello');
?>

A HELLO

B HELLO, FRIEND!

C Hello, friend!

D Hello

💡 Hint

Remember that filters run in the order they are added, and each filter receives the output of the previous one.

The first filter appends ', friend!' to 'Hello', making 'Hello, friend!'. The second filter converts the entire string to uppercase, resulting in 'HELLO, FRIEND!'.

📝 Syntax

intermediate

Which option causes a syntax error in adding a filter?

Which of the following filter hook additions will cause a PHP syntax error?

A add_filter('title', function($title) { return strtoupper($title); });

B add_filter('title', fn($title) => strtoupper($title));

C add_filter('title', function($title) { return strtoupper($title);

D add_filter('title', 'strtoupper');

💡 Hint

Check for missing punctuation or braces in the function syntax.

Option C is missing the closing semicolon and closing brace for the anonymous function, causing a syntax error.

❓ state_output

advanced

What is the value of $content after applying these filters?

Given the following code, what is the value of $content after apply_filters('content_filter', $content)?

Wordpress

<?php
$content = 'Start';
add_filter('content_filter', function($text) {
    return $text . ' + First';
});
add_filter('content_filter', function($text) {
    return $text . ' + Second';
}, 5);
$content = apply_filters('content_filter', $content);
?>

A Start + Second + First

B Start + First + Second

C Start + First

D Start + Second

💡 Hint

Lower priority numbers run earlier in WordPress filters.

The filter with priority 5 runs before the default priority 10. So ' + Second' is added first, then ' + First'.

🔧 Debug

advanced

Which option causes a runtime error when applying this filter?

Given this filter hook, which option will cause a runtime error when apply_filters('number_filter', 10) is called?

Wordpress

<?php
add_filter('number_filter', function($num) {
    return $num * 2;
});

A add_filter('number_filter', function($num) { return $num + 5; });

B add_filter('number_filter', function($num) { return $num / 2; });

C add_filter('number_filter', function($num) { return $num - 3; });

D add_filter('number_filter', function($num) { return $num->value; });

💡 Hint

Consider the type of the input and what operations are valid on it.

Option D tries to access a property on an integer, causing a fatal error.

🧠 Conceptual

expert

Which option best describes how filter hooks modify data in WordPress?

Select the option that correctly explains how filter hooks work in WordPress.

A Filters let you modify data by passing it through a chain of functions that each return a modified value.

B Filters allow you to replace core WordPress functions with your own implementations.

C Filters automatically cache data to improve WordPress performance without code changes.

D Filters are used to enqueue scripts and styles in WordPress themes and plugins.

💡 Hint

Think about how data flows through filters and how the output changes.

Filters pass data through multiple functions, each can modify and return the data, allowing flexible changes without altering core code.