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

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

Builds Unique ID for storage and retrieval.


The old way to serialize the callback caused issues and this function is the solution. It works by checking for objects and creating a new property in the class to keep track of the object and new objects of the same class that need to be added.

It also allows for the removal of actions and filters for objects after they change class properties. It is possible to include the property $wp_filter_id in your class and set it to "null" or a number to bypass the workaround.
However this will prevent you from adding new classes and any new classes will overwrite the previous hook by the same class.

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

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$hook_name string Required
Unused. The name of the filter to build ID for.
$callback callable|string|array Required
The callback to generate ID for. The callback may or may not exist.
$priority int Required
Unused. The order in which the functions associated with a particular action are executed.

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string Unique function ID for usage as array key.

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File: wp-includes/plugin.php. View all references

function _wp_filter_build_unique_id( $hook_name, $callback, $priority ) {
	if ( is_string( $callback ) ) {
		return $callback;

	if ( is_object( $callback ) ) {
		// Closures are currently implemented as objects.
		$callback = array( $callback, '' );
	} else {
		$callback = (array) $callback;

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

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Version Description
5.3.0 Removed workarounds for spl_object_hash().
$hook_name and $priority are no longer used, and the function always returns a string.
2.2.3 Introduced.

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