bbp_validate_reply_to( int $reply_to, int $reply_id )

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

	// The parent reply must actually be a reply
	if ( ! bbp_is_reply( $reply_to ) ) {
		$reply_to = 0;
	}

	// The parent reply cannot be itself
	if ( $reply_id === $reply_to ) {
		$reply_to = 0;
	}

	return (int) $reply_to;
}