Hook priority and arguments in Wordpress - Step-by-Step Execution

Concept Flow - Hook priority and arguments

Add Hook with Priority

↓

WordPress Stores Hook

↓

Trigger Hook Event

↓

WordPress Sorts Hooks by Priority

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Call Hook Functions in Order

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Pass Arguments to Hook Functions

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Functions Execute with Arguments

WordPress stores hooks with their priority and arguments, then calls them in order passing the arguments when the hook runs.

Execution Sample

Wordpress

<?php
add_action('init', 'first_hook', 10, 2);
add_action('init', 'second_hook', 5, 1);
function first_hook($arg1, $arg2) { echo "First: $arg1, $arg2"; }
function second_hook($arg1) { echo "Second: $arg1"; }
do_action('init', 'hello', 'world');
?>

This code adds two functions to the 'init' hook with different priorities and argument counts, then triggers the hook with two arguments.

Execution Table

Step	Action	Hook Name	Priority	Arguments Passed	Function Called	Output
1	Add Hook	init	10	2	first_hook	none
2	Add Hook	init	5	1	second_hook	none
3	Trigger Hook	init	N/A	['hello', 'world']	N/A	none
4	Sort Hooks by Priority	init	5 then 10	N/A	N/A	none
5	Call Hook Function	init	5	['hello']	second_hook	Second: hello
6	Call Hook Function	init	10	['hello', 'world']	first_hook	First: hello, world
7	End	init	N/A	N/A	N/A	All hooks called

💡 All hooks for 'init' called in priority order; lower number runs first.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 4	After Step 6	Final
Hooks['init']	empty	[{func: first_hook, priority:10, args:2}]	[{func: first_hook, priority:10, args:2}, {func: second_hook, priority:5, args:1}]	[{func: second_hook, priority:5, args:1}, {func: first_hook, priority:10, args:2}]	N/A	N/A
Arguments Passed	N/A	N/A	N/A	N/A	['hello', 'world']	N/A

Key Moments - 3 Insights

Why does 'second_hook' run before 'first_hook' even though it was added later?

Because WordPress runs hooks in order of priority number, lower numbers run first. 'second_hook' has priority 5, which is before 10 of 'first_hook' (see execution_table steps 4 and 5).

Why does 'second_hook' only get one argument even though two were passed?

Because when adding the hook, it was set to accept only 1 argument. WordPress passes only as many arguments as the hook expects (see execution_table step 5).

What happens if two hooks have the same priority?

They run in the order they were added. WordPress sorts by priority first, then by order added (not shown in this example).

Visual Quiz - 3 Questions

Look at the execution table, which hook runs first when 'init' is triggered?

A first_hook

B second_hook

C both run simultaneously

D none run

💡 Hint

Check step 4 and 5 where hooks are sorted and called by priority.

At which step does WordPress sort hooks by priority?

A Step 4

B Step 3

C Step 5

D Step 6

💡 Hint

Look for the step mentioning sorting hooks by priority.

If 'first_hook' was added with priority 3, how would the execution order change?

A They run in the order added

B second_hook runs before first_hook

C first_hook runs before second_hook

D Neither runs

💡 Hint

Lower priority number means earlier execution (see concept_flow and execution_table).

Concept Snapshot

WordPress hooks run in order of priority (lower number first).
Add hooks with add_action or add_filter specifying priority and argument count.
When hook triggers, WordPress sorts hooks by priority.
Each hook function receives only the number of arguments it expects.
Hooks with same priority run in order added.

Full Transcript

In WordPress, hooks are functions attached to events. When you add a hook, you specify its priority and how many arguments it accepts. WordPress stores these hooks and when the event triggers, it sorts hooks by priority number, running lower numbers first. Then it calls each hook function, passing the arguments given by the event, but only as many as the hook expects. This way, you control the order and data each hook receives. For example, if you add two hooks to 'init' with priorities 10 and 5, the one with priority 5 runs first. If the event passes two arguments but a hook expects only one, it gets just one. Hooks with the same priority run in the order they were added.

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