function private_str($str, $start, $end){
$after = mb_substr($str, 0, $start, 'utf8');
$repeat = str_repeat('*', $end);
$before = mb_substr($str, ($start + $end), strlen($str), 'utf8');
return $after.$repeat.$before;
}

<?php echo private_str($ipaddress, 1, 8); ?>