When You use
header("location:url");
in PHP ... On server it Display warning and page does not redirect.
Here is the solutiion
<?php
ob_start();
....
logic
.........
ob_end_flush();
?>
OR
Try replacing header("Location:order_sell.php"); with
echo '<META HTTP-EQUIV="Refresh" Content="0; URL=filename.php">';
exit;