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function is not described

registered_meta_key_exists() WP 4.6.0

Checks if a meta key is registered.

No Hooks.

Return

true/false. True if the meta key is registered to the object type and, if provided, the object subtype. False if not.

Usage

registered_meta_key_exists( $object_type, $meta_key, $object_subtype );
$object_type(string) (required)
The type of object.
$meta_key(string) (required)
The meta key.
$object_subtype(string)
The subtype of the object type.
Default: ''

Changelog

Since 4.6.0 Introduced.
Since 4.9.8 The $object_subtype parameter was added.

Code of registered meta key exists: wp-includes/meta.php WP 5.2.2

<?php
function registered_meta_key_exists( $object_type, $meta_key, $object_subtype = '' ) {
	$meta_keys = get_registered_meta_keys( $object_type, $object_subtype );

	return isset( $meta_keys[ $meta_key ] );
}