Title: _wp_filter_build_unique_id
Published: April 25, 2014
Last modified: August 20, 2026

---

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

## In this article

 * [Description](https://developer.wordpress.org/reference/functions/_wp_filter_build_unique_id/?output_format=md#description)
 * [Parameters](https://developer.wordpress.org/reference/functions/_wp_filter_build_unique_id/?output_format=md#parameters)
 * [Return](https://developer.wordpress.org/reference/functions/_wp_filter_build_unique_id/?output_format=md#return)
 * [Source](https://developer.wordpress.org/reference/functions/_wp_filter_build_unique_id/?output_format=md#source)
 * [Related](https://developer.wordpress.org/reference/functions/_wp_filter_build_unique_id/?output_format=md#related)
 * [Changelog](https://developer.wordpress.org/reference/functions/_wp_filter_build_unique_id/?output_format=md#changelog)

[ Back to top](https://developer.wordpress.org/reference/functions/_wp_filter_build_unique_id/?output_format=md#wp--skip-link--target)

This function’s access is marked private. This means it is not intended for use 
by plugin or theme developers, only by core. It is listed here for completeness.

Builds a unique string ID for a hook callback function.

## 󠀁[Description](https://developer.wordpress.org/reference/functions/_wp_filter_build_unique_id/?output_format=md#description)󠁿

Functions and static method callbacks are just returned as strings and shouldn’t
have any speed penalty.

## 󠀁[Parameters](https://developer.wordpress.org/reference/functions/_wp_filter_build_unique_id/?output_format=md#parameters)󠁿

 `$hook_name`stringrequired

Unused. The name of the filter to build ID for.

`$callback`callablerequired

The callback to generate ID for. The callback may or may not exist.

`$priority`intrequired

Unused. The order in which the functions associated with a particular action are
executed.

## 󠀁[Return](https://developer.wordpress.org/reference/functions/_wp_filter_build_unique_id/?output_format=md#return)󠁿

 string|null Unique function ID for usage as array key, or null if it couldn’t be
determined.

## 󠀁[Source](https://developer.wordpress.org/reference/functions/_wp_filter_build_unique_id/?output_format=md#source)󠁿

    ```php
    function _wp_filter_build_unique_id( $hook_name, $callback, $priority ): ?string {
    	if ( is_string( $callback ) ) {
    		return $callback;
    	}

    	if ( is_object( $callback ) ) {
    		return (string) spl_object_id( $callback );
    	}

    	if ( ! isset( $callback[1] ) || ! is_string( $callback[1] ) ) {
    		return null;
    	}

    	if ( is_object( $callback[0] ) ) {
    		// Object class calling.
    		return ( (string) spl_object_id( $callback[0] ) ) . $callback[1];
    	} elseif ( is_string( $callback[0] ) ) {
    		// Static calling.
    		return $callback[0] . '::' . $callback[1];
    	}

    	return null;
    }
    ```

[View all references](https://developer.wordpress.org/reference/files/wp-includes/plugin.php/)
[View on Trac](https://core.trac.wordpress.org/browser/tags/7.1/src/wp-includes/plugin.php#L1002)
[View on GitHub](https://github.com/WordPress/wordpress-develop/blob/7.1/src/wp-includes/plugin.php#L1002-L1024)

## 󠀁[Related](https://developer.wordpress.org/reference/functions/_wp_filter_build_unique_id/?output_format=md#related)󠁿

| Used by | Description | 
| [WP_Hook::add_filter()](https://developer.wordpress.org/reference/classes/wp_hook/add_filter/)`wp-includes/class-wp-hook.php` |

Adds a callback function to a filter hook.

  | 
| [WP_Hook::remove_filter()](https://developer.wordpress.org/reference/classes/wp_hook/remove_filter/)`wp-includes/class-wp-hook.php` |

Removes a callback function from a filter hook.

  | 
| [WP_Hook::has_filter()](https://developer.wordpress.org/reference/classes/wp_hook/has_filter/)`wp-includes/class-wp-hook.php` |

Checks if a specific callback has been registered for this hook.

  |

## 󠀁[Changelog](https://developer.wordpress.org/reference/functions/_wp_filter_build_unique_id/?output_format=md#changelog)󠁿

| Version | Description | 
| [7.1.0](https://developer.wordpress.org/reference/since/7.1.0/) | Uses spl_object_id() instead of spl_object_hash() for performance. | 
| [6.9.0](https://developer.wordpress.org/reference/since/6.9.0/) | Returns explicit null if an invalid callback is supplied. | 
| [5.3.0](https://developer.wordpress.org/reference/since/5.3.0/) | Removed workarounds for spl_object_hash().
 `$hook_name` and `$priority` are no longer used, and no longer returns false, but can still return void for invalid callbacks. | 
| [2.2.3](https://developer.wordpress.org/reference/since/2.2.3/) | Introduced. |

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