it appears that gethostbyname() fails if the domain name contains unicode. Example:
$ host 10.10.10.128
128.10.10.10.in-addr.arpa domain name pointer PC-de-S\130bastien.flexi.robbak.com.
$ php
<?php echo gethostbyaddr( '10.10.10.128' ); ?>
10.10.10.128
gethostbyaddr is listed as unicode compatable in version 6, so a fix may be in the works.