You could sort the array after the difference using sort().
$array1 = array("a" =>'1','2','3','4','5','6','7','8','9','10');
$array2 = array("b" => '1','3','4','6','7','8','10');
$diff = array_diff($array1, $array2);
sort($diff);
http://codepad.viper-7.com/yREvAg
Or like others are writing, you could use the array_values()