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Once you know which type of hook you need, the basic pattern is simple: define a callback, register it with add_action() or add_filter(), then verify the hook’s priority and argument list in its reference.
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What a WordPress hook is and why it matters
A hook is an extension point built into WordPress, a theme, or a plugin. At that point, the original code either announces that something is happening or passes a value through a changeable pipeline. Other code can connect to that point with a callback function or method.
This design lets you change behavior from a plugin or a theme extension instead of modifying WordPress core files. Core edits are fragile: an update can overwrite them, and the change becomes difficult to disable or move. A hook keeps your customization separate and gives other components a predictable place to integrate.
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WordPress has two hook contracts:
- Actions signal an event or execution point. Your callback does work there, such as printing markup, saving data, sending a notification, or triggering another process. The return value from an action callback is not used as replacement data.
- Filters pass a value through your callback. Your function receives that value, optionally receives additional arguments, changes it, and must return the result for the next callback or the calling code.
The hook name is a string such as wp_footer or the_title. The name alone does not tell you the complete contract: the hook reference determines whether it is an action or filter, when it fires, which arguments it supplies, and what those arguments contain.
Actions and filters compared
| Question | Action | Filter |
|---|---|---|
| Primary purpose | Perform work at a defined point | Transform a value during execution |
| Callback return value | Not used by the action hook | Required; returned value continues through the filter chain |
| Typical output | Output, persistence, notifications, or another task | A revised title, text string, setting, or other passed value |
| Side effects | Expected when the task requires them | Keep the callback focused on the value; avoid direct output or unrelated global-state changes |
| Registration function | add_action() |
add_filter() |
Use an action when the requirement sounds like “do this when WordPress reaches this point.” Use a filter when it sounds like “change this value before it is used or displayed.” Choosing the wrong contract is a common reason for code that appears to run but has no effect.
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An action callback can be a named function or a method. The callback should have a unique prefix so it does not collide with another plugin or theme.
Basic action example
function wporg_show_message() {
echo '<p>Hello from an action.</p>';
}
add_action( 'wp_footer', 'wporg_show_message' );When WordPress fires wp_footer, it calls wporg_show_message(), which prints a paragraph. The function does not return a replacement value because actions do not consume callback return values.
Where to put the code
For a site-specific change, put the registration in a small custom plugin or in an appropriate theme extension point. A plugin keeps the behavior active when the site changes themes. Avoid placing customizations in WordPress core files. Keep the callback narrowly scoped and load it before the hook is fired; code added after an event has already run cannot respond to that occurrence.
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Action callbacks with arguments
Some actions provide one or more arguments. Your callback must declare only the values you intend to use, and the registration must state how many arguments WordPress should pass. Confirm the action reference before choosing that number rather than assuming every hook supplies the same data.
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A filter callback receives the current value, changes it, and returns the result. Returning the value is not optional: if the callback returns nothing, the next code in the chain receives an unusable result.
Basic filter example
function wporg_add_suffix( $title ) {
return $title . ' – My site';
}
add_filter( 'the_title', 'wporg_add_suffix' );Whenever the_title is processed, the callback appends text and returns the new title. It does not echo markup or alter unrelated global state. Those side effects belong in an action or in a separately designed operation.
Filters with more than one argument
function wporg_adjust_title( $title, $post_id ) {
return $title . ' #' . $post_id;
}
add_filter( 'the_title', 'wporg_adjust_title', 12, 2 );The third argument, 12, is the priority. The fourth argument, 2, tells WordPress to pass two arguments to the callback. The first argument is normally the value being filtered; the other arguments are hook-specific context. The callback signature and the accepted-argument count must agree with the hook reference.
Priority controls execution order
When several callbacks use the same hook, WordPress orders them by priority. The default priority is 10. A callback at priority 9 runs before one at 10; a callback at 11 runs after it. If callbacks share a priority, they run in the order in which they were registered.
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| Registration | Resulting order |
|---|---|
add_filter( 'the_title', 'first', 9 ); |
Runs before callbacks at 10 or higher |
add_filter( 'the_title', 'default_callback' ); |
Runs at the default priority of 10 |
add_filter( 'the_title', 'last', 11 ); |
Runs after callbacks at 10 or lower |
Priority is useful when one callback must see another callback’s output or when your code must run before a later task. It is not a substitute for understanding the data contract. First confirm the hook and arguments, then choose the smallest priority adjustment that gives the required order.
Accepted arguments: match the hook’s contract
The fourth parameter of add_action() or add_filter() is the number of arguments WordPress should pass to your callback. A callback that declares two parameters should be registered to accept two arguments when the hook supplies them. Conversely, do not declare or request arguments that the hook does not provide.
Use the hook reference to verify:
- Whether the hook is an action or a filter.
- The order and meaning of every argument.
- The maximum argument count available at that point.
- When the hook fires in the request or rendering lifecycle.
This check prevents warnings, missing context, and accidental use of the wrong value. It also makes your code resilient when several callbacks are attached to the same hook.
How hooks run during a request
An action is fired with do_action(). WordPress reaches that statement and calls each registered callback in priority order. The callbacks may print, store, calculate, or trigger other work, but their return values do not replace anything in the action call.
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This difference explains a practical rule: echoing inside a filter does not modify the value being filtered, while failing to return a value can break the filter chain. Keep output-oriented tasks on actions and transformation logic on filters.
Create custom hooks in a plugin
If you maintain a plugin, expose extension points so other developers can add behavior without editing your files. Use do_action() for an event and apply_filters() for a value that should be alterable.
do_action( 'wporg_before_settings' );
$value = apply_filters( 'wporg_settings_label', 'Settings' );Other code can subscribe to the custom action or filter:
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function wporg_log_settings_start() {
// Perform a task before the settings area is rendered.
}
add_action( 'wporg_before_settings', 'wporg_log_settings_start' );
function wporg_change_settings_label( $label ) {
return $label . ' & options';
}
add_filter( 'wporg_settings_label', 'wporg_change_settings_label' );Give custom hooks a distinctive prefix, document their argument order, and place them at a stable point in your plugin’s flow. For text or other data sent to the browser, a filter is generally the more flexible choice because another component can alter the value before output.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Remove an action or filter safely
Use remove_action() or remove_filter() when you need to detach a callback that was registered elsewhere. Removal works only when you identify the original hook and callable and use the same priority. The removal call must run after the original registration has occurred.
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function wporg_show_message() {
echo '<p>Hello from an action.</p>';
}
add_action( 'wp_footer', 'wporg_show_message', 10 );
remove_action( 'wp_footer', 'wporg_show_message', 10 );For methods, the callable identity must match the original method and object instance (or the same static method form). A different anonymous function, a different object, or a different priority is a different callback as far as removal is concerned. If the original code has not registered yet, your removal does nothing; place it at a later, reliable extension point.
A safe beginner workflow
- State the desired change. Decide whether you need to perform a task or transform a value.
- Find the exact hook. Confirm its name, type, timing, supplied arguments, and expected value.
- Create a unique callback. Prefix the function or method name for your project and keep its responsibility narrow.
- Register it explicitly. Use
add_action()oradd_filter(); specify priority and accepted arguments only when needed. - Load it in the right place. Put the code in a plugin or suitable theme extension and ensure it loads before the hook fires.
- Test the contract. For an action, verify the task occurs. For a filter, verify the returned value reaches the caller and remains valid.
- Adjust order only when necessary. Change priority after confirming that another callback’s order is the actual cause.
Common failures and how to correct them
The callback never runs
Check that the plugin or theme code is active, the hook name is spelled exactly, and the registration executes before the hook fires. A correct callback attached too late cannot respond to an event that has already occurred.
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The filter has no visible effect
Make sure the callback returns the changed value and that you used add_filter(), not add_action(). Also verify that the code path actually applies the filter you selected; not every title, setting, or output string passes through the same hook.
Arguments are missing or warnings appear
Compare the callback signature and the fourth registration argument with the hook reference. Request only the arguments supplied by that hook and put them in the documented order.
Another callback overwrites your result
Inspect priorities. A later filter can replace the value your callback returned, so move your callback earlier or later only after identifying the ordering requirement. For actions, a later callback may print additional output or perform a conflicting task.
Removing a callback changes nothing
Match the original hook name, callable, and priority, and run the removal after registration. With methods, use the same callable form and object identity. Anonymous callbacks that you cannot reference later are difficult or impossible to remove cleanly.
A filter causes unexpected output or side effects
Keep filters focused on transforming their input and returning it. Move direct output, database writes, notifications, or unrelated global changes to an action or another dedicated operation.
Choosing the right hook at a glance
- Need to print a footer message? Attach an action to the footer event.
- Need to append text to a title? Attach a filter and return the revised title.
- Need another plugin to react before your settings screen? Expose a custom action with
do_action(). - Need another plugin to rename a label? Expose a custom filter with
apply_filters()and assign its returned value. - Need to disable an existing callback? Identify its exact callable and priority, then remove it after registration.
Hooks are small interfaces, but their contracts matter. Treat the hook type, argument list, priority, callback identity, and execution timing as part of the API. That discipline lets you extend WordPress cleanly while keeping custom behavior separate from core code.
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