Why doesn't the redirect work? How would you fix it?

if ($stmtselect->rowCount() > 0) {
    header("refresh:5;url=https://www.websitename.com");
    $_SESSION['accounts'] = $user;
    echo 'You have signed in successfully!';
    die();
}else {
    header("refresh:5;url=index.php");
    echo 'Incorrect Username or Password or Email';
    die();
}

I have the header first so why doesn't it work? Just so you know, this page is a form. It submits stuff to me. If it works then I want it to redirect after the echo.

I get this error.

jquery-3.3.1.min.js:2 [Violation] 'load' handler took 1365ms