สอบถามเรื่อง login หน่อยค่ะ Warning: session_start

<?php
if( isset($_POST['login']) ) 
{ 
require('connect/connect.php');

$username = mysqli_real_escape_string($connect, $_POST['username']);
$password = mysqli_real_escape_string($connect, $_POST['password']);
$sql = "SELECT * FROM member WHERE member_username = '".$username."' and member_password = '".$password."'";
$query = mysqli_query($connect, $sql) ;
$result = mysqli_fetch_array($query);

if( !$result )
{
   echo '<script>alert( "ชื่อผู้ใช้งานและรหัสผ่านไม่ถูกต้อง" );window.location = "userlogin.php";</script>'; die();
} 
else 
{	 
	session_start();
	
    $_SESSION["id"] = $result["member_id"];
    $_SESSION["username"] = $result["member_username"];
    $_SESSION["name"] = $result["member_name"];
    $_SESSION["surname"] = $result["member_surname"];
 
	session_write_close();
	
	echo '<script>window.location = "index.php";</script>'; die();

}
 
   mysqli_close($connect);  
} 

?>