acck/lib/validhostname.php

<?php
function validhostname($hostname,$pattmods='') {
	preg_replace('/./u','.',$hostname,-1,$c);
	(strlen($hostname)>$c) ? $multibyte=true : $multibyte=false;
	if ($multibyte) $hostname=idn_to_ascii($hostname,IDNA_DEFAULT,INTL_IDNA_VARIANT_UTS46);
	if (strlen($hostname)>253)
		return false;
	$labs=explode('.',$hostname);
	foreach ($labs as $lab) {
		$len=strlen($lab);
		if ($len==0 || $len>63)
			return false;
		if (preg_match('#^[a-z0-9]([a-z0-9-]*[a-z0-9])?$#i'.$pattmods,$lab)!==1)
			return false;
	}
	return true;
}
?>