Hook priority and arguments in Wordpress - Mini Project: Build & Apply

Hook Priority and Arguments in WordPress

📖 Scenario: You are building a simple WordPress plugin that modifies the footer text. You want to learn how to use hooks with different priorities and how to pass and use arguments in your hooked functions.

🎯 Goal: Create a WordPress plugin that uses the add_filter hook to change the footer text. You will set up the hook with different priorities and handle arguments passed to the hooked function.

📋 What You'll Learn

Create a function that modifies the footer text

Use add_filter to hook the function to admin_footer_text

Set a custom priority for the hook

Make the hooked function accept and use arguments passed by the hook

💡 Why This Matters

🌍 Real World

WordPress developers often customize themes and plugins by using hooks with specific priorities and arguments to control how and when their code runs.

💼 Career

Understanding hook priority and arguments is essential for WordPress plugin and theme development jobs, enabling developers to write clean, compatible, and maintainable code.

1

Create the footer text function

Create a function called custom_footer_text that takes one argument called $text and returns the string 'Custom footer text.'

Wordpress

# Your code here

Hint

Define a function with one parameter and return the exact string.

2

Hook the function to admin_footer_text with default priority

Use add_filter to hook the function custom_footer_text to the admin_footer_text hook with the default priority and specify that it accepts 1 argument.

Wordpress

<?php
function custom_footer_text($text) {
    return 'Custom footer text.';
}

# Your code here

Hint

Use add_filter with priority 10 and 1 argument.

3

Change the hook priority to 20

Modify the add_filter call to set the priority to 20 while keeping the function name custom_footer_text, hook admin_footer_text, and 1 argument.

Wordpress

<?php
function custom_footer_text($text) {
    return 'Custom footer text.';
}

# Your code here

Hint

Change the priority number to 20 in the add_filter call.

4

Modify the function to append text using the argument

Change the custom_footer_text function so it appends the string ' - Powered by WordPress' to the $text argument and returns the combined string.

Wordpress

<?php
function custom_footer_text($text) {
    # Your code here
}

add_filter('admin_footer_text', 'custom_footer_text', 20, 1);

Hint

Use string concatenation to add the extra text to the argument.

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