php - How can I make substr work with utf-8?

Why is my text not displayed properly when I use "substr"?

function limitstring($input,$number) {
    if (strlen($input) > $number){
        $input = substr($input, 0, $number) . '...';
    }
    return $input;
}

Input:

echo $row['text'];

Output: Käse

Input:

limitstring($row['text'],60);

Output:
K�se...