registered_meta_key_exists example

function unregister_meta_key( $object_type, $meta_key, $object_subtype = '' ) {
    global $wp_meta_keys;

    if ( ! registered_meta_key_exists( $object_type, $meta_key, $object_subtype ) ) {
        return false;
    }

    $args = $wp_meta_keys[ $object_type ][ $object_subtype ][ $meta_key ];

    if ( isset( $args['sanitize_callback'] ) && is_callable( $args['sanitize_callback'] ) ) {
        if ( ! empty( $object_subtype ) ) {
            remove_filter( "sanitize_{$object_type}_meta_{$meta_key}_for_{$object_subtype}", $args['sanitize_callback'] );
        } else {
            remove_filter( "sanitize_{$object_type}_meta_{$meta_key}", $args['sanitize_callback'] );
        }
    }