When using UTF-8 and need to convert to uppercase with 
special characters like the german ä,ö,ü (didn't test for french,polish,russian but think it should work, too) try this:
function strtoupper_utf8($string){
 $string=utf8_decode($string);
 $string=strtoupper($string);
 $string=utf8_encode($string);
 return $string;
}