WordPress Hooks: Actions and Filters for PHP Developers

Hooks are WordPress’s main way of letting your code join in. Instead of editing WordPress core, you say “when this happens, call my function”.

If you remember Callback Functions, hooks are the same idea: WordPress keeps a list of callbacks for each named hook and calls them at the right time.

Two kinds

	Action	Filter
Meaning	“Something happened. Do something.”	“Here is a value. Change it and give it back.”
Your function	Does work (add a script, send an email)	Returns a modified value
Registered with	`add_action()`	`add_filter()`
WordPress calls	`do_action()`	`apply_filters()`

A tiny model of how hooks work

This plain PHP version shows what happens behind the scenes.

index.php

PHP

<?php
$filters = [];

function my_add_filter(array &$filters, string $name, callable $fn): void
{
    $filters[$name][] = $fn;
}

function my_apply_filters(array $filters, string $name, mixed $value): mixed
{
    foreach ($filters[$name] ?? [] as $fn) {
        $value = $fn($value);
    }

    return $value;
}

my_add_filter($filters, "title", fn($t) => trim($t));
my_add_filter($filters, "title", fn($t) => strtoupper($t));

echo my_apply_filters($filters, "title", "  hello hooks  ");

Output

OUTPUT

HELLO HOOKS

WordPress stores callbacks in a list per hook name and runs them in order. The real functions add priorities, argument counts and more.

The real thing

index.php

PHP

<?php
add_filter("demo_title", fn($t) => trim($t));
add_filter("demo_title", fn($t) => strtoupper($t));

echo apply_filters("demo_title", "  hello hooks  ");

Output

OUTPUT

HELLO HOOKS

Hook names and where they fire

WordPress runs thousands of hooks. A few you will meet early:

Hook	Type	When
`init`	action	WordPress has loaded, before output. Register post types here
`wp_enqueue_scripts`	action	The place to add front-end CSS and JS
`admin_menu`	action	Add admin menu pages
`save_post`	action	A post was saved
`the_content`	filter	The post content is about to be printed
`the_title`	filter	A title is about to be printed
`body_class`	filter	The list of `<body>` classes

Priorities and arguments

index.php

PHP

<?php
add_action("demo_hook", function () { echo "second "; }, 20);
add_action("demo_hook", function () { echo "first "; }, 5);
add_action("demo_hook", function (string $name) { echo "hello $name"; }, 30, 1);

do_action("demo_hook", "reader");

Output

OUTPUT

first second hello reader

The third argument of add_action() is the priority (lower runs earlier, default 10). The fourth is how many arguments the callback accepts.

Callbacks: functions, closures and methods

index.php

PHP

<?php
add_action("init", "my_plugin_setup");                      // a function name
add_action("init", function () { /* ... */ });              // a closure
add_action("init", [$this, "setup"]);                       // an object method
add_action("init", [self::class, "staticMethod"]);          // a static method
add_action("init", $this->setup(...));                      // first-class callable (PHP 8.1+)

Closures cannot be removed later with remove_action(), because you have no name to give. Use named functions or methods when other code might need to unhook yours.

Remove a hook

index.php

PHP

<?php
function noisy(): void { echo "noisy "; }

add_action("demo_remove", "noisy");
remove_action("demo_remove", "noisy");

do_action("demo_remove");
echo has_action("demo_remove", "noisy") === false ? "removed" : "still there";

Output

OUTPUT

removed

Your own hooks

You can add hooks to your own plugin or theme, so other developers (or you later) can extend it. See Actions and Filters.

Why hooks matter

Updating WordPress does not erase your changes, because they live in your plugin or theme.
Many plugins can change the same thing, each adding its own callback.
Order is controlled with priorities.

info

This is the key idea of WordPress PHP

Once you understand “register a callback for a named moment”, most of WordPress development makes sense.