bbp_update_reply_to( int $reply_id, int $reply_to )

function bbp_update_reply_to( $reply_id = 0, $reply_to = 0 ) {

	// Validation
	$reply_id = bbp_get_reply_id( $reply_id );
	$reply_to = bbp_validate_reply_to( $reply_to );

	// Update or delete the `reply_to` postmeta
	if ( ! empty( $reply_id ) ) {

		// Update the reply to
		if ( !empty( $reply_to ) ) {
			update_post_meta( $reply_id, '_bbp_reply_to', $reply_to );

		// Delete the reply to
		} else {
			delete_post_meta( $reply_id, '_bbp_reply_to' );
		}
	}

	return (int) apply_filters( 'bbp_update_reply_to', (int) $reply_to, $reply_id );
}