Hook Functions

Last updated: February 12, 2026

Core functions for registering and executing WordPress hooks.

Source: wp-includes/plugin.php

Filter Functions

add_filter()

Adds a callback function to a filter hook.

add_filter( 
    string   $hook_name, 
    callable $callback, 
    int      $priority = 10, 
    int      $accepted_args = 1 
): true

Since: 0.71

Parameters:

Name	Type	Default	Description
`$hook_name`	string	—	The name of the filter to add the callback to
`$callback`	callable	—	The callback to run when the filter is applied
`$priority`	int	10	Execution order (lower = earlier)
`$accepted_args`	int	1	Number of arguments the callback accepts

Returns: Always returns true. Does not validate callback existence.

Examples:

// Basic usage
add_filter( 'the_title', 'my_title_filter' );

function my_title_filter( $title ) {
    return '🔥 ' . $title;
}

// With priority (run early)
add_filter( 'the_content', 'my_early_filter', 5 );

// Multiple arguments
add_filter( 'the_title', 'my_title_context', 10, 2 );

function my_title_context( $title, $post_id ) {
    return $title . ' (Post #' . $post_id . ')';
}

// Anonymous function
add_filter( 'body_class', function( $classes ) {
    $classes[] = 'my-custom-class';
    return $classes;
} );

// Class method
add_filter( 'the_content', array( $this, 'filter_content' ) );
add_filter( 'the_content', array( 'My_Class', 'static_method' ) );
add_filter( 'the_content', 'My_Class::static_method' );

apply_filters()

Calls all callbacks registered to a filter hook.

apply_filters( string $hook_name, mixed $value, mixed ...$args ): mixed

Since: 0.71
Since: 6.0.0 — Formalized ...$args parameter

Parameters:

Name	Type	Description
`$hook_name`	string	The name of the filter hook
`$value`	mixed	The value to filter
`...$args`	mixed	Additional arguments passed to callbacks

Returns: The filtered value after all callbacks are applied.

Examples:

// Basic usage
$title = apply_filters( 'the_title', $post->post_title, $post->ID );

// Creating custom filter points
$message = apply_filters( 'my_plugin_message', 'Default message', $context );

// Chained filters
$content = apply_filters( 'the_content', $raw_content );
// Each registered callback receives the output of the previous one

apply_filters_ref_array()

Calls filter callbacks with arguments passed as an array.

apply_filters_ref_array( string $hook_name, array $args ): mixed

Since: 3.0.0

Parameters:

Name	Type	Description
`$hook_name`	string	The name of the filter hook
`$args`	array	Arguments to pass (first element is the value to filter)

Returns: The filtered value.

Example:

$args = array( $value, $arg1, $arg2, $arg3 );
$result = apply_filters_ref_array( 'my_filter', $args );

has_filter()

Checks if any callbacks are registered for a filter hook.

has_filter( 
    string                      $hook_name, 
    callable|string|array|false $callback = false,
    int|false                   $priority = false 
): bool|int

Since: 2.5.0
Since: 6.9.0 — Added $priority parameter

Parameters:

Name	Type	Default	Description
`$hook_name`	string	—	The name of the filter hook
`$callback`	callable\|false	false	Specific callback to check for
`$priority`	int\|false	false	Specific priority to check

Returns:

No $callback: true if anything registered, false otherwise
With $callback: Priority (int) if found, false if not found
With both: true/false for that specific callback at that priority

Warning: Return value can be 0 (valid priority). Use === for comparison.

Examples:

// Check if hook has any callbacks
if ( has_filter( 'the_content' ) ) {
    // Something is filtering content
}

// Check for specific callback (returns priority or false)
$priority = has_filter( 'the_content', 'wpautop' );
if ( false !== $priority ) {
    echo "wpautop is at priority $priority";
}

// Check callback at specific priority
if ( has_filter( 'the_content', 'wpautop', 10 ) ) {
    // wpautop is registered at priority 10
}

remove_filter()

Removes a callback from a filter hook.

remove_filter( 
    string                $hook_name, 
    callable|string|array $callback, 
    int                   $priority = 10 
): bool

Since: 1.2.0

Parameters:

Name	Type	Default	Description
`$hook_name`	string	—	The filter hook
`$callback`	callable	—	The callback to remove
`$priority`	int	10	Must match the priority used when adding

Returns: true if removed, false if callback wasn’t found.

Important: The $callback and $priority must match exactly what was passed to add_filter().

Examples:

// Remove named function
remove_filter( 'the_content', 'wpautop' );

// Remove class method (must have same instance reference)
remove_filter( 'the_content', array( $instance, 'method' ), 10 );

// Remove static method
remove_filter( 'the_content', array( 'My_Class', 'method' ), 10 );

// Removing closures requires storing reference
$my_closure = function( $content ) { return $content; };
add_filter( 'the_content', $my_closure );
// Later...
remove_filter( 'the_content', $my_closure );

remove_all_filters()

Removes all callbacks from a filter hook.

remove_all_filters( string $hook_name, int|false $priority = false ): true

Since: 2.7.0

Parameters:

Name	Type	Default	Description
`$hook_name`	string	—	The filter hook
`$priority`	int\|false	false	Remove only at this priority, or all if false

Returns: Always true.

Examples:

// Remove all callbacks from the_content
remove_all_filters( 'the_content' );

// Remove only priority 10 callbacks
remove_all_filters( 'the_content', 10 );

current_filter()

Gets the name of the currently executing filter hook.

current_filter(): string|false

Since: 2.5.0

Returns: Hook name, or false if no hook is executing.

Example:

function my_flexible_callback( $value ) {
    $hook = current_filter();
    
    if ( 'the_title' === $hook ) {
        return strtoupper( $value );
    }
    
    return $value;
}

add_filter( 'the_title', 'my_flexible_callback' );
add_filter( 'the_excerpt', 'my_flexible_callback' );

doing_filter()

Checks if a specific filter is currently being executed.

doing_filter( string|null $hook_name = null ): bool

Since: 3.9.0

Parameters:

Name	Type	Default	Description
`$hook_name`	string\|null	null	Hook to check, or null for any

Returns: true if the filter is in the execution stack.

Note: Unlike current_filter(), this checks the entire stack, not just the most recent hook.

Example:

function my_callback() {
    if ( doing_filter( 'the_content' ) ) {
        // We're somewhere in the_content filter chain
        // (may be nested inside another callback)
    }
}

did_filter()

Gets the number of times a filter has been applied.

did_filter( string $hook_name ): int

Since: 6.1.0

Parameters:

Name	Type	Description
`$hook_name`	string	The filter hook name

Returns: Number of times the filter was applied during the current request.

Example:

$count = did_filter( 'the_content' );
echo "the_content has been applied $count times";

Action Functions

add_action()

Adds a callback function to an action hook.

add_action( 
    string   $hook_name, 
    callable $callback, 
    int      $priority = 10, 
    int      $accepted_args = 1 
): true

Since: 1.2.0

Internally calls add_filter(). Parameters and behavior are identical.

Examples:

// Basic usage
add_action( 'init', 'my_init_function' );

// With priority
add_action( 'wp_head', 'my_head_content', 5 );

// With arguments
add_action( 'save_post', 'my_save_handler', 10, 3 );

function my_save_handler( $post_id, $post, $update ) {
    if ( $update ) {
        // Post was updated, not created
    }
}

do_action()

Executes all callbacks registered to an action hook.

do_action( string $hook_name, mixed ...$arg ): void

Since: 1.2.0
Since: 5.3.0 — Formalized ...$arg parameter

Parameters:

Name	Type	Description
`$hook_name`	string	The action hook name
`...$arg`	mixed	Arguments passed to callbacks

Returns: Nothing (void).

Examples:

// Trigger WordPress core action
do_action( 'init' );

// Create custom action point
do_action( 'my_plugin_loaded', $plugin_instance );

// With multiple arguments
do_action( 'my_plugin_event', $arg1, $arg2, $arg3 );

do_action_ref_array()

Executes action callbacks with arguments passed as an array.

do_action_ref_array( string $hook_name, array $args ): void

Since: 2.1.0

Parameters:

Name	Type	Description
`$hook_name`	string	The action hook name
`$args`	array	Arguments to pass to callbacks

Example:

$args = array( $post_id, $post, $update );
do_action_ref_array( 'save_post', $args );

has_action()

Checks if any callbacks are registered for an action hook.

has_action( 
    string                      $hook_name, 
    callable|string|array|false $callback = false,
    int|false                   $priority = false 
): bool|int

Since: 2.5.0
Since: 6.9.0 — Added $priority parameter

Alias for has_filter(). Parameters and return values are identical.

remove_action()

Removes a callback from an action hook.

remove_action( 
    string                $hook_name, 
    callable|string|array $callback, 
    int                   $priority = 10 
): bool

Since: 1.2.0

Alias for remove_filter(). Parameters and return values are identical.

remove_all_actions()

Removes all callbacks from an action hook.

remove_all_actions( string $hook_name, int|false $priority = false ): true

Since: 2.7.0

Alias for remove_all_filters().

current_action()

Gets the name of the currently executing action hook.

current_action(): string|false

Since: 3.9.0

Alias for current_filter().

doing_action()

Checks if a specific action is currently being executed.

doing_action( string|null $hook_name = null ): bool

Since: 3.9.0

Alias for doing_filter().

did_action()

Gets the number of times an action has been fired.

did_action( string $hook_name ): int

Since: 2.1.0

Parameters:

Name	Type	Description
`$hook_name`	string	The action hook name

Returns: Number of times the action was fired during the current request.

Example:

if ( did_action( 'init' ) ) {
    // init has already fired
}

// Check if fired exactly once
if ( 1 === did_action( 'plugins_loaded' ) ) {
    // First time plugins_loaded has run
}

Deprecated Hook Functions

apply_filters_deprecated()

Fires a deprecated filter with a deprecation notice.

apply_filters_deprecated( 
    string $hook_name, 
    array  $args, 
    string $version, 
    string $replacement = '', 
    string $message = '' 
): mixed

Since: 4.6.0

Parameters:

Name	Type	Default	Description
`$hook_name`	string	—	Deprecated hook name
`$args`	array	—	Arguments (first element is filtered value)
`$version`	string	—	Version when deprecated
`$replacement`	string	”	Replacement hook name
`$message`	string	”	Additional message

Example:

// Old code
return apply_filters( 'old_filter', $value, $extra_arg );

// Deprecated replacement
return apply_filters_deprecated( 
    'old_filter', 
    array( $value, $extra_arg ), 
    '5.0.0', 
    'new_filter' 
);

do_action_deprecated()

Fires a deprecated action with a deprecation notice.

do_action_deprecated( 
    string $hook_name, 
    array  $args, 
    string $version, 
    string $replacement = '', 
    string $message = '' 
): void

Since: 4.6.0

Example:

// Old code
do_action( 'old_action', $arg1, $arg2 );

// Deprecated replacement
do_action_deprecated( 
    'old_action', 
    array( $arg1, $arg2 ), 
    '5.0.0', 
    'new_action',
    'The old_action hook is no longer used.'
);

Internal Functions

_wp_call_all_hook()

Processes callbacks on the ‘all’ hook.

_wp_call_all_hook( array $args ): void

Since: 2.5.0
Access: Private

Called internally by apply_filters() and do_action() when the ‘all’ hook has callbacks.

_wp_filter_build_unique_id()

Generates a unique string ID for a callback.

_wp_filter_build_unique_id( 
    string                $hook_name, 
    callable|string|array $callback, 
    int                   $priority 
): string|null

Since: 2.2.3
Since: 5.3.0 — Removed workarounds, always returns string
Access: Private

Returns:

Callback Type	Return Value
String function	`"function_name"`
Static method	`"Class::method"`
Object method	`spl_object_hash($obj) . "method"`
Closure	`spl_object_hash($closure) . ""`
Invalid	`null`

Plugin Lifecycle Hooks

register_activation_hook()

Sets the activation hook for a plugin.

register_activation_hook( string $file, callable $callback ): void

Since: 2.0.0

Parameters:

Name	Type	Description
`$file`	string	Main plugin file path (use `__FILE__`)
`$callback`	callable	Function to run on activation

Example:

register_activation_hook( __FILE__, 'my_plugin_activate' );

function my_plugin_activate() {
    add_option( 'my_plugin_version', '1.0.0' );
    flush_rewrite_rules();
}

register_deactivation_hook()

Sets the deactivation hook for a plugin.

register_deactivation_hook( string $file, callable $callback ): void

Since: 2.0.0

Example:

register_deactivation_hook( __FILE__, 'my_plugin_deactivate' );

function my_plugin_deactivate() {
    wp_clear_scheduled_hook( 'my_plugin_cron' );
    flush_rewrite_rules();
}

register_uninstall_hook()

Sets the uninstall hook for a plugin.

register_uninstall_hook( string $file, callable $callback ): void

Since: 2.7.0

Important: Callback must be a static method or function (not an instance method).

Alternative: Create uninstall.php in plugin root directory. Check for WP_UNINSTALL_PLUGIN constant.

Example:

register_uninstall_hook( __FILE__, 'My_Plugin::uninstall' );

class My_Plugin {
    public static function uninstall() {
        delete_option( 'my_plugin_settings' );
        // Clean up database tables, etc.
    }
}

Hook Functions

Filter Functions

add_filter()

apply_filters()

apply_filters_ref_array()

has_filter()

remove_filter()

remove_all_filters()

current_filter()

doing_filter()

did_filter()

Action Functions

add_action()

do_action()

do_action_ref_array()

has_action()

remove_action()

remove_all_actions()

current_action()

doing_action()

did_action()

Deprecated Hook Functions

apply_filters_deprecated()

do_action_deprecated()

Internal Functions

_wp_call_all_hook()

_wp_filter_build_unique_id()

Plugin Lifecycle Hooks

register_activation_hook()

register_deactivation_hook()

register_uninstall_hook()

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