add_action and add_filter in Wordpress - Step-by-Step Execution

Concept Flow - add_action and add_filter

WordPress loads core

↓

Hooks registered with add_action/add_filter

↓

WordPress runs event or filter point

↓

Check for hooked functions

↓

No

↓

Skip

↓

If filter, modify data and return

↓

Continue WordPress execution

WordPress loads and registers hooks with add_action or add_filter. When an event or filter point runs, WordPress checks for hooked functions and runs them in order. Filters modify data and return it.

Execution Sample

Wordpress

<?php
add_action('init', 'say_hello');
function say_hello() {
  echo 'Hello!';
}
?>

This code hooks the say_hello function to the 'init' action, so 'Hello!' prints when WordPress initializes.

Execution Table

Step	Hook Point	Hooked Function	Action Taken	Output/Result
1	WordPress loads	add_action('init', 'say_hello')	Registers say_hello to 'init'	No output yet
2	WordPress reaches 'init' action	say_hello	Runs say_hello function	Prints 'Hello!'
3	No more hooked functions	N/A	Continue normal WordPress flow	Normal page loads

💡 All hooked functions for 'init' run; WordPress continues normal execution

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
hook_registry['init']	empty	['say_hello']	['say_hello']	['say_hello']
output_buffer	empty	empty	'Hello!'	'Hello!'

Key Moments - 2 Insights

Why doesn't 'Hello!' print immediately after add_action is called?

Because add_action only registers the function to run later at the 'init' hook point. The function runs only when WordPress reaches that hook (see execution_table step 2).

What is the difference between add_action and add_filter?

add_action runs functions at specific points without changing data, while add_filter runs functions that receive data, modify it, and return it. Filters change data flow; actions do not (see concept_flow).

Visual Quiz - 3 Questions

Look at the execution table, what is the output after step 2?

A Error message

B No output

C 'Hello!'

D 'init'

💡 Hint

Check the 'Output/Result' column at step 2 in the execution_table

At which step does WordPress run the hooked function?

A Step 2

B Step 1

C Step 3

D Never

💡 Hint

Look at the 'Action Taken' column to see when the function runs

If you use add_filter instead of add_action, what changes?

A The function runs earlier

B The hooked function can modify and return data

C The function runs multiple times automatically

D Nothing changes

💡 Hint

Refer to the concept_flow description about filters modifying data

Concept Snapshot

add_action('hook_name', 'function_name')
- Registers a function to run at a WordPress event
- Function runs when WordPress reaches that hook

add_filter('hook_name', 'function_name')
- Registers a function to modify data
- Function receives data, returns modified data

Hooks run in order of registration
Functions hooked do not run immediately

Full Transcript

In WordPress, add_action and add_filter let you connect your functions to specific points called hooks. add_action runs your function when WordPress hits an event, like 'init'. add_filter runs your function to change data passed through a filter. The process starts with WordPress loading and registering your hooks. When WordPress reaches the hook point, it runs all functions registered there. Actions run functions without changing data, filters run functions that receive and return data. For example, add_action('init', 'say_hello') registers say_hello to run at 'init'. When WordPress runs 'init', it calls say_hello, which prints 'Hello!'. This happens after registration, not immediately. Understanding when and how your hooked functions run helps you customize WordPress behavior safely and effectively.