mb_internal_encoding("UTF-8");
$array1 = array('aümg', 'abc', 'aécas', 'bdö', 'äsasf', 'aäfa');
uasort($array1, 'utf_8_german::cmp');
var_dump($array3);
Result: array(6) { [5]=> string(5) "aäfa" [1]=> string(3) "abc" [2]=> string(6) "aécas" [0]=> string(5) "aümg" [4]=> string(6) "äsasf" [3]=> string(4) "bdö" }

class utf_8_german{
    // everything else is sorted at the end
    static $order = '0123456789AaÄäBbCcDdEeéFfGgHhIiJjKkLlMm
NnOoÖöPpQqRrSsßTtUuÜüVvWwXxYyZz';
    static $char2order;
    static function cmp($a, $b) {
        if($a == $b){
            return 0;
        }
        if (empty(self::$char2order)){
            $order = 1;
            $len = mb_strlen(self::$order);
            for ($order=0; $order<$len; ++$order){
                self::$char2order[mb_substr(self::$order, $order, 1)] = $order;
            }
        }
        $len_a = mb_strlen($a);
        $len_b = mb_strlen($b);
        $max=min($len_a, $len_b);
        for($i=0; $i<$max; ++$i){
            $char_a= mb_substr($a, $i, 1);
            $char_b= mb_substr($b, $i, 1);
            if ($char_a == $char_b) continue;
            $order_a = (isset(self::$char2order[$char_a])) ? self::$char2order[$char_a] : 9999;
            $order_b = (isset(self::$char2order[$char_b])) ? self::$char2order[$char_b] : 9999;
            return ($order_a < $order_b) ? -1 : 1;
        }
        return ($len_a < $len_b) ? -1 : 1;
    }
}