You are giving multiple Content-Type headers. application/vnd.ms-excel is enough.
And there are couple of syntax error too. To statement termination with ; on the echo statement and wrong filename extension.
header("Content-Type: application/vnd.ms-excel; charset=utf-8");
header("Content-Disposition: attachment; filename=abc.xls"); //File name extension was wrong
header("Expires: 0");
header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
header("Cache-Control: private",false);
echo "Some Text"; //no ending ; here