ใช้ PHP mysqli แล้วใช้คำสั่ง Header ไม่ได้ครับ ขึ้น Warning: Cannot modify header information - headers already sent by....

	session_start();
	ini_set('display_errors',1);
	error_reporting(~0);

	$servername = "localhost";
	$username = "admin";
	$userpassword = "1234";
	$dbname = "data";
	$conn = mysqli_connect($servername,$username,$userpassword,$dbname);
	$conn->query("set names utf8");

	//ตรวจรูปแบบ email
	if (!preg_match('/^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,3})$/',$_POST["email"])){ 
		$_SESSION["email"]='invalid';
		session_write_close();
		header("location:http://website.com/");
		exit(0);
	}