create Thumbnail on POST

i use your usefull thumbnail_images.class, and it works!
but if wanna create thumbnails on POST and after this redirect to a summary-page. if i include your class then the redirect wont work, whitout it works!

my code:
if ((isset($_POST["MM_insert"])) && ($_POST["MM_insert"] == "save")) {
require_once("../thumbnail_images.class.php");

//Thumb_PDF

$thumb1 = new thumbnail_images();
$thumb1 -> PathImgOld = "../../files/".$pic;
$thumb1 -> PathImgNew = "../../files/thumb_".$pic;
$thumb1 -> NewWidth = 45;
$thumb1 -> NewHeight = 60;
$thumb1 -> create_thumbnail_images();

$insertGoTo = "check.php";

header(sprintf("Location: %s", $insertGoTo));
}

why i does the $insertGoTo not work whit your class?
thanks a lot for help