After installing zabbix 1.8.1 frontend, I've started frontend configuration. On database configuration page, I've configured database user password as za$$iXuseR. After conclude configuration It's not possible to log on zabbix frontend. The message presented is "Undefined variable: iXuseR[/var/www/html/zabbix/conf/zabbix.conf.php:28]". I think there is a problem with double "$" interpretation in /var/www/html/zabbix/conf/zabbix.conf.php file. I've solved temporally this problem by modifying manually the file /var/www/html/zabbix/conf/zabbix.conf.php and quoting the database user password with single quotes (the default is double quotes).
$DB["PASSWORD"] = "za$$iXuseR";
$DB["PASSWORD"] = 'za$$iXuseR';