Hook priority and arguments in Wordpress - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output order of hooks with different priorities?

Consider these WordPress hooks added to the same action 'init' with different priorities. What will be the order of output when the 'init' action runs?

Wordpress

<?php
add_action('init', function() { echo 'First'; }, 10);
add_action('init', function() { echo 'Second'; }, 5);
add_action('init', function() { echo 'Third'; }, 15);
do_action('init');
?>

A SecondThirdFirst

B SecondFirstThird

C ThirdSecondFirst

D FirstSecondThird

💡 Hint

Lower priority numbers run earlier in WordPress hooks.

Hooks with lower priority numbers run before those with higher numbers. Here, priority 5 runs first, then 10, then 15.

❓ state_output

intermediate

How many arguments does the hooked function receive?

Given this code, how many arguments will the function 'my_callback' receive when the 'save_post' action runs?

Wordpress

<?php
function my_callback($post_id, $post, $update) {
    // ...
}
add_action('save_post', 'my_callback', 10, 3);
do_action('save_post', 123, (object) ['ID' => 123], true);
?>

A 3

B 2

C 1

D 0

💡 Hint

The fourth parameter in add_action controls how many arguments are passed.

The 'add_action' call specifies 3 arguments to pass, so the function receives all three.

📝 Syntax

advanced

Which option causes a syntax error in hook registration?

Which of these add_action calls will cause a syntax error?

A add_action('init', 'my_function', 10);

B add_action('init', 'my_function', 10, 2);

C add_action('init', 'my_function' 10, 2);

D add_action('init', 'my_function');

💡 Hint

Check for missing commas between arguments.

Option C is missing a comma between 'my_function' and 10, causing a syntax error.

🔧 Debug

advanced

Why does the hooked function not receive all arguments?

Given this code, why does 'my_callback' only receive 1 argument instead of 3?

Wordpress

<?php
function my_callback($a, $b, $c) {
    var_dump(func_get_args());
}
add_action('some_hook', 'my_callback', 10, 1);
do_action('some_hook', 1, 2, 3);
?>

A Because add_action specifies only 1 argument to pass

B Because do_action only passes 1 argument

C Because the function definition is incorrect

D Because the hook name is wrong

💡 Hint

Check the fourth parameter of add_action.

The fourth parameter of add_action controls how many arguments are passed to the callback. Here it is 1, so only one argument is passed.

🧠 Conceptual

expert

What happens if two hooks have the same priority and modify the same data?

If two functions hooked to the same filter have the same priority and both modify the input data, what determines which modification applies first?

A WordPress merges the modifications automatically

B The hook added last runs first, so its modification applies first

C Hooks with the same priority run in random order

D The hook added first runs first, so its modification applies first

💡 Hint

Think about how WordPress stores hooks internally.

When hooks have the same priority, WordPress runs them in the order they were added. The first added runs first.

Practice

1. What does the priority number control when adding a function to a WordPress hook using add_action or add_filter?

easy

A. The order in which functions run on the same hook

B. The number of arguments the function accepts

C. The type of hook (action or filter)

D. The name of the hook to attach to

Solution

1. Step 1: Understand the role of priority in hooks

   Priority determines the sequence functions run when multiple functions are attached to the same hook.

2. Step 2: Differentiate priority from other parameters

   Priority is not about argument count or hook type; it only controls execution order.

3. Final Answer:

   The order in which functions run on the same hook -> Option A

4. Quick Check:

   Priority controls execution order = B [OK]

Hint: Priority sets function run order on hooks [OK]

Common Mistakes:

• Confusing priority with argument count
• Thinking priority changes hook type
• Assuming priority sets hook name

2. Which of the following is the correct syntax to add a function my_function to the init action hook with priority 15 and accepting 2 arguments?

easy

A. add_action('init', 'my_function', 2);

B. add_action('init', 'my_function', 2, 15);

C. add_action('init', my_function, 15, 2);

D. add_action('init', 'my_function', 15, 2);

Solution

1. Step 1: Recall the parameter order of add_action

   The order is hook name, function name (string), priority (default 10), and accepted arguments (default 1).

2. Step 2: Match parameters to the question

   Priority is 15, accepted arguments is 2, so the call is add_action('init', 'my_function', 15, 2);

3. Final Answer:

   add_action('init', 'my_function', 15, 2); -> Option D

4. Quick Check:

   Priority=15, args=2 syntax = D [OK]

Hint: Remember add_action(hook, func, priority, args) order [OK]

Common Mistakes:

• Swapping priority and argument count
• Omitting quotes around function name
• Passing wrong number of parameters

3. Given the code below, what will be the output?

function first_func($arg) {
  return $arg . ' first';
}
function second_func($arg) {
  return $arg . ' second';
}
add_filter('test_hook', 'first_func', 20, 1);
add_filter('test_hook', 'second_func', 10, 1);
$result = apply_filters('test_hook', 'Run');
echo $result;

medium

A. Run second

B. Run first second

C. Run second first

D. Run first

Solution

1. Step 1: Understand filter execution order by priority

   Lower priority numbers run first. Here, second_func has priority 10, first_func has 20.

2. Step 2: Trace filter application

   Input 'Run' goes to second_func first, returning 'Run second'. Then 'Run second' goes to first_func, returning 'Run second first'.

3. Final Answer:

   Run second first -> Option C

4. Quick Check:

   Lower priority runs first = A [OK]

Hint: Lower priority number runs first in filters [OK]

Common Mistakes:

• Assuming higher priority runs first
• Ignoring filter chaining order
• Confusing actions and filters

4. What is wrong with this code snippet?

function my_filter($a, $b) {
  return $a + $b;
}
add_filter('calc', 'my_filter', 10, 1);
$result = apply_filters('calc', 5, 10);
echo $result;

medium

A. The priority number 10 is invalid

B. The function expects 2 arguments but only 1 is accepted in add_filter

C. apply_filters cannot pass more than one argument

D. Function name should be in double quotes

Solution

1. Step 1: Check function parameter count vs accepted args

   Function my_filter expects 2 arguments, but add_filter specifies only 1 argument accepted.

2. Step 2: Understand consequence of argument mismatch

   Only 1 argument will be passed, so $b will be undefined, causing incorrect behavior or error.

3. Final Answer:

   The function expects 2 arguments but only 1 is accepted in add_filter -> Option B

4. Quick Check:

   Accepted args must match function params = A [OK]

Hint: Match accepted args to function parameters [OK]

Common Mistakes:

• Ignoring argument count mismatch
• Thinking priority 10 is invalid
• Believing apply_filters limits arguments

5. You want to add two functions to the same filter hook modify_text. The first function should run before the second and both accept 3 arguments. Which is the correct way to add them?

hard

A. add_filter('modify_text', 'first_func', 5, 3); add_filter('modify_text', 'second_func', 10, 3);

B. add_filter('modify_text', 'first_func', 10, 3); add_filter('modify_text', 'second_func', 5, 3);

C. add_filter('modify_text', 'first_func', 3, 5); add_filter('modify_text', 'second_func', 3, 10);

D. add_filter('modify_text', 'first_func', 3); add_filter('modify_text', 'second_func', 3);

Solution

1. Step 1: Determine priority order for execution

   Lower priority number runs first, so first_func should have lower priority than second_func.

2. Step 2: Set accepted arguments correctly

   Both functions accept 3 arguments, so the fourth parameter in add_filter must be 3 for both.

3. Final Answer:

   add_filter('modify_text', 'first_func', 5, 3); add_filter('modify_text', 'second_func', 10, 3); -> Option A

4. Quick Check:

   Lower priority runs first, args=3 = C [OK]

Hint: Lower priority runs first; set args count correctly [OK]

Common Mistakes:

• Swapping priority numbers
• Setting wrong accepted argument count
• Omitting accepted arguments parameter