Common filter hooks in Wordpress - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a filter modifying post title

What will be the output of the following WordPress code snippet when the post title is displayed?

Wordpress

<?php
function add_prefix_to_title($title) {
    return 'Prefix: ' . $title;
}
add_filter('the_title', 'add_prefix_to_title');
// Later in template
echo apply_filters('the_title', 'Hello World');
?>

A Prefix Hello World

B Hello World

C the_title

D Prefix: Hello World

💡 Hint

Remember that filters modify the value passed through them.

The filter 'the_title' is hooked with a function that adds 'Prefix: ' before the title. When apply_filters is called, it returns the modified string.

❓ component_behavior

intermediate

Effect of removing a filter hook

Given the following code, what will be the output of the post content?

Wordpress

<?php
function add_suffix_to_content($content) {
    return $content . ' - Suffix';
}
add_filter('the_content', 'add_suffix_to_content');
remove_filter('the_content', 'add_suffix_to_content');
echo apply_filters('the_content', 'Sample content');
?>

A Sample content

B Sample content - Suffix - Suffix

C - Suffix

D Sample content - Suffix

💡 Hint

Consider what happens when a filter is removed before applying it.

The filter is added and then immediately removed, so when apply_filters runs, no modification happens.

📝 Syntax

advanced

Correct syntax to add a filter with priority

Which option correctly adds a filter to 'the_excerpt' with a priority of 15 and accepts 2 arguments?

A add_filter('the_excerpt', 'modify_excerpt', 2, 15);

B add_filter('the_excerpt', 'modify_excerpt', 15, 2);

C add_filter('the_excerpt', 'modify_excerpt', 15);

D add_filter('the_excerpt', 15, 'modify_excerpt', 2);

💡 Hint

Remember the order of parameters in add_filter is: hook, callback, priority, accepted_args.

The correct order is hook name, callback function, priority number, and number of accepted arguments.

🔧 Debug

advanced

Why does this filter not modify the output?

Consider this code snippet: Why does the output remain 'Hello' instead of 'HELLO'?

A apply_filters is not the correct function to use.

B The filter hook name 'the_title' is incorrect.

C The function does not return the modified title.

D The function should use echo instead of return.

💡 Hint

Filters must return the modified value to change output.

The function changes the variable but does not return it, so the filter returns null and original value is used.

🧠 Conceptual

expert

Understanding filter hook execution order

If two filters are added to 'the_content' hook as follows: add_filter('the_content', 'first_filter', 20); add_filter('the_content', 'second_filter', 10); And both functions append their name to the content string, what will be the output of apply_filters('the_content', 'Text')?

Wordpress

<?php
function first_filter($content) {
    return $content . ' first_filter';
}
function second_filter($content) {
    return $content . ' second_filter';
}
add_filter('the_content', 'first_filter', 20);
add_filter('the_content', 'second_filter', 10);
echo apply_filters('the_content', 'Text');
?>

A Text second_filter first_filter

B Text second_filter

C Text first_filter second_filter

D Text first_filter

💡 Hint

Lower priority numbers run earlier in the filter chain.

Filters with lower priority run first, so second_filter runs before first_filter, appending its text first.