mb_convert_encoding($str, 'UTF-8', 'auto');

$file = 'images/да так 1.jpg';//this is in UTF-8, needs to be system encoding (Russian)
$new_filename = mb_convert_encoding($file, "Windows-1251", "utf-8");//turn utf-8 to system encoding Windows-1251 (Russian)

$new_filename = mb_convert_encoding($file, "utf-8", "Windows-1251");