When you're not sure, how the current locale is set, you might find the following function useful. It's strtolower for utf8-formatted text:
<?php
function strtolower_utf8($inputString) {
$outputString = utf8_decode($inputString);
$outputString = strtolower($outputString);
$outputString = utf8_encode($outputString);
return $outputString;
}
?>
It's not suitable for every occasion, but it surely gets in handy. I use it for lowering German 'Umlauts' like ä and ö.